So kind of the same as Spielberg, Scott, and Lucas doing each other special favours? Good enough reason than any I guess.
I would imagine that if the D Pad support is there, it would be pretty awkward to play using teh GameCube D Pad, due to it's tiny size. But then, some people might like thumb blisters! Lol. I never really had a problem playing Super Smash Bros Melee with the GameCube pad though, and would probably be my choice of controller, so long as the controls remain pretty much the same