Ease of use? A lot of good secondary characters are easy to use at a basic level, MK being no exception. MK, Game and Watch, Falco, D3, and to some extent Wario aren't all particularly hard to use. If your argument is that you want to focus more time on your main, all of these characters are quick pick-ups, especially Falco and D3.
MK is the safest CP character, but not always the best.
Assume that:
First Match: You stay your main, you win
Second Match: You don't know who they're going to counterpick, so MK is the safest option
Third Match: Your main/counterpick depending on who they be, you win
First Match: You stay your main, you lose
Second Match: You counterpick
Third Match: You don't know who they'll counterpick, so MK is the safest option.
On paper, MK is the safest option for one game out of your 3 game set. However, whether or not counterpicking will give you a huge benefit depends on who you main, who your opponent mains, and who your opponent IS. You play against Anther? He's going to go Pikachu. There's no point in thinking, "LOLS WUT IF HE GOES AGAINST THE GUY I SUCK AGAINST???" and pick MK, when you know he's a Pikachu main and he's going to be Pikachu. This goes for really any person in your region. You should know at least half of the mains of the people you're facing at a tournament.
Plus, if you main a character who has at worst a 60:40 match-up, counterpicking isn't too beneficial. You're either riding an uphill battle against a bad match-up with a character you know, or you're playing a beneficial match-up with a character you aren't nearly as good with. As long as your character doesn't get completely destroyed by someone else (Wolf, ROB, DK) there's no reason to need a secondary.
So while MK is the safest option, using a secondary in general isn't always.