Yes, it is true. His 4-5% throws make Marth deadly even when he's not using his sword. There are far too many Marth's who don't use his grab game to its full potential. It's probably one of the best in the cast because they put your opponent in a disadvantageous position where you are able to easily follow it up in a multitude of ways. His grab range is also still pretty decent, especially pivot grabs.
A lot of Marth's tend to shy away from Marth's grabs because it requires you to go inside his sword range. I urge you to understand that this is not as bad as it sounds. Getting the grab can be as simple as dashing into a shield grab. If Marth dashes towards you suddenly what is your first reaction in the middle of a match? Shield probably, as Marth can either jump into an aerial or dancing blade. They want to be able to punish this. Maybe they will spot dodge, pivot grab ***** this. Grabbing is the ultimate weapon against shields and Marth can force your opponent into shielding with little difficulty.
Landing
Even top players are going to land on the stage with an air dodge and/or shield some of the time. It's just a natural habit a lot of us have when we are trying to avoid an attack to be sent out again. Using a smash attack in this situation is possible, but it is not as reliable as our other option. Grab. Pivot grab.. whatever. Just grab them in this situation.
Punishing
Example: You shield an ftilt by Ike. You could easily dancing blade in this situation, but you can also just dash grab him. Why grab him instead of just using dancing blade? A few reasons. One is to refresh your moves a tad (grab jabs+the actual throw), but the more important one is you can follow up your grab(s) to do even more damage. Maybe you just shielded a dsmash by MK. The MK, fearing the inevitable dancing blade shields. You grab him instead (especially if you were late on your shield drop). I'm definitely not saying do this every time, since the extra damage isn't guaranteed at all (unless of course you are grab releasing MK). It's just another way to get the grab.
There are so many ways to get your opponent scared and have them shield. Short hop at your opponent.. what may be their first reaction? Oh.. he will probably space a fair. Fast fall your empty short hop and grab them. Dtilt at your opponent and watch them shield or roll, dash grab. Forward b 1, grab. They don't even have to shield... You can literally punish MK's dash approaches with a pivot grab. There's many ways and I'm not going to list them all. You will find more as you get more experienced in competitive play.
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Now that that's covered... How exactly do you follow up the grabs? Which grab do you use in certain situations?
Stage Positioning
One of the best properties of Marth's grabs. They are able to put your opponent in a bad position where you can further apply pressure with your sword. If you grab your opponent near the edge don't be a moron and throw him into center stage (the ONLY time this is OK is if you are at 0% or so and are chaining it to an fsmash). Throw him off stage and try to keep him there for as long as possible. If he isn't smart he may try to be too aggressive in his getting back to stage and just double jump right back into you. Punish this. Punish it with a smash, dancing blade, or just grab again.. punish him. They should be grabbing the ledge almost every time in this situation because that's their safest option, but some people aren't smart enough to realize this. Also, if you are center stage simply fthrow or dthrow to corner them and apply pressure.
Now I'm going to go over each throw and a few little things you can do to follow them up...
F-Throw/D-Throw (Putting these together because they have basically the same follow ups)
"Fthrow is my tech chase" - Shaya
No kidding. A lot of the time your opponent will simply air dodge after the fthrow. Just dash grab him again. The situation is reset and you get more chip damage. Maybe they won't, perhaps you can chain it to a fair or nair, even an fsmash if their DI is bad enough. It's a guessing game, really. There's definitely more than this, but I'm sure you all can figure out your own little things, yes? And as well know these two chain into fsmashes.
B-Throw
...
LOL
Don't use this. It doesn't send them at a lower trajectory, that vid of Ninjalink you all saw was simply bad DI. I tested it. In all situations you can bthrow you should just dthrow. More damage, sexier looking, less lag. WTF? This is Marth's worst move.
Up-Throw
You all should use this throw more. Not just on Snake.. basically everyone. Even if you up throw at like 5%, they may just air dodge on their way down and you can grab them again. Marth's juggling game is amazing and this is a one way ticket set up. Not much to say, but this throw is good.
Grab Releasing
This stuff is good, none of it is guaranteed unless it's Ness/Lucas. I don't recommend being too risky and just randomly throwing out a dsmash. Dtilt is much safer and can pressure their shield well if that is what they do once released. You can also simply grab them again if they do this, if you condition them to not shield THEN maybe you can dsmash... but seriously it's not very reliable. You have sooo many options from the grab release, experiment with it. Also, grab releasing them on the ledge is also recommended instead of just throwing, they fall low and they have less of a chance to recover. If you grab them on the very edge of the stage I recommend just not jabbing them at all and wait for their jump release, some characters have a very hard time coming back from the trajectory it sends them.
Not going to go over jump releases, we have threads dedicated to that.
Well that about wraps it up.. Marth's grab game should be abused by everyone. The traps you put your opponent into are hard to get out of. Many mindgames can be explored with this. I may add more to this .. I may have rushed a section or two.
Either way... discuss.
A lot of Marth's tend to shy away from Marth's grabs because it requires you to go inside his sword range. I urge you to understand that this is not as bad as it sounds. Getting the grab can be as simple as dashing into a shield grab. If Marth dashes towards you suddenly what is your first reaction in the middle of a match? Shield probably, as Marth can either jump into an aerial or dancing blade. They want to be able to punish this. Maybe they will spot dodge, pivot grab ***** this. Grabbing is the ultimate weapon against shields and Marth can force your opponent into shielding with little difficulty.
Landing
Even top players are going to land on the stage with an air dodge and/or shield some of the time. It's just a natural habit a lot of us have when we are trying to avoid an attack to be sent out again. Using a smash attack in this situation is possible, but it is not as reliable as our other option. Grab. Pivot grab.. whatever. Just grab them in this situation.
Punishing
Example: You shield an ftilt by Ike. You could easily dancing blade in this situation, but you can also just dash grab him. Why grab him instead of just using dancing blade? A few reasons. One is to refresh your moves a tad (grab jabs+the actual throw), but the more important one is you can follow up your grab(s) to do even more damage. Maybe you just shielded a dsmash by MK. The MK, fearing the inevitable dancing blade shields. You grab him instead (especially if you were late on your shield drop). I'm definitely not saying do this every time, since the extra damage isn't guaranteed at all (unless of course you are grab releasing MK). It's just another way to get the grab.
There are so many ways to get your opponent scared and have them shield. Short hop at your opponent.. what may be their first reaction? Oh.. he will probably space a fair. Fast fall your empty short hop and grab them. Dtilt at your opponent and watch them shield or roll, dash grab. Forward b 1, grab. They don't even have to shield... You can literally punish MK's dash approaches with a pivot grab. There's many ways and I'm not going to list them all. You will find more as you get more experienced in competitive play.
----------------
Now that that's covered... How exactly do you follow up the grabs? Which grab do you use in certain situations?
Stage Positioning
One of the best properties of Marth's grabs. They are able to put your opponent in a bad position where you can further apply pressure with your sword. If you grab your opponent near the edge don't be a moron and throw him into center stage (the ONLY time this is OK is if you are at 0% or so and are chaining it to an fsmash). Throw him off stage and try to keep him there for as long as possible. If he isn't smart he may try to be too aggressive in his getting back to stage and just double jump right back into you. Punish this. Punish it with a smash, dancing blade, or just grab again.. punish him. They should be grabbing the ledge almost every time in this situation because that's their safest option, but some people aren't smart enough to realize this. Also, if you are center stage simply fthrow or dthrow to corner them and apply pressure.
Now I'm going to go over each throw and a few little things you can do to follow them up...
F-Throw/D-Throw (Putting these together because they have basically the same follow ups)
"Fthrow is my tech chase" - Shaya
No kidding. A lot of the time your opponent will simply air dodge after the fthrow. Just dash grab him again. The situation is reset and you get more chip damage. Maybe they won't, perhaps you can chain it to a fair or nair, even an fsmash if their DI is bad enough. It's a guessing game, really. There's definitely more than this, but I'm sure you all can figure out your own little things, yes? And as well know these two chain into fsmashes.
B-Throw
...
LOL
Don't use this. It doesn't send them at a lower trajectory, that vid of Ninjalink you all saw was simply bad DI. I tested it. In all situations you can bthrow you should just dthrow. More damage, sexier looking, less lag. WTF? This is Marth's worst move.
Up-Throw
You all should use this throw more. Not just on Snake.. basically everyone. Even if you up throw at like 5%, they may just air dodge on their way down and you can grab them again. Marth's juggling game is amazing and this is a one way ticket set up. Not much to say, but this throw is good.
Grab Releasing
This stuff is good, none of it is guaranteed unless it's Ness/Lucas. I don't recommend being too risky and just randomly throwing out a dsmash. Dtilt is much safer and can pressure their shield well if that is what they do once released. You can also simply grab them again if they do this, if you condition them to not shield THEN maybe you can dsmash... but seriously it's not very reliable. You have sooo many options from the grab release, experiment with it. Also, grab releasing them on the ledge is also recommended instead of just throwing, they fall low and they have less of a chance to recover. If you grab them on the very edge of the stage I recommend just not jabbing them at all and wait for their jump release, some characters have a very hard time coming back from the trajectory it sends them.
Not going to go over jump releases, we have threads dedicated to that.
Well that about wraps it up.. Marth's grab game should be abused by everyone. The traps you put your opponent into are hard to get out of. Many mindgames can be explored with this. I may add more to this .. I may have rushed a section or two.
Either way... discuss.
steel 
