Kirby's throws are all good, and have different uses. I don't like that F throw gives such less percent than the other throws, but it still is good because of its function. As for U throw, yes many would have liked for U throw to do more damage and yes many would have liked it to be a kill throw, but the move still functions well.
1. D throw: This one seems to be the easiest for most Kirby players to apply well. You can get a good tech chase game going and at higher percents against certain characters it will grant you a combo usually leading into a kill.
2. B throw: The main use of this throw is for positioning. This throw works well to get them off the stage due to its trajectory.
3. U throw: At certain percents it can combo. Against fast fallers you can use U throw at killing percents since D throw won't allow you to get a combo at killing percents against them. If they DI away you can combo into air hammer at around 70% which can kill.
4. F throw: Unlike all the other throws where they can actually get behind you, F throw will always send them in front of you despite their DI. Since you are barely still airborne after the move you can perform cutter dash to chase them instantly after the throw (the recovery is almost instant so the cutter dash needs to be input as early as possible.) After hitting with a cutter dash you should be able to continue the combo with another aerial attack. If they DI towards you instead of away, then you can easily hit with a different aerial attack. This move does not combo well at higher percents.