Goofy uses a shield because he hates weapons. It says this in the instruction booklet. Don't you kids read those things anymore?
Donald and Goofy were alright, they each had their own uses at some times(Donald was best for a welltimed heal, while Goofy could plow over some guys and throw a potion when you needed it). I wish you could bring guys like Jack with you to other worlds, though.
I hope they make magic actually useful this go-round.