Except the GameCube controller with the GameCube adapter is
currently the only truly wired controller option for the Wii U. If you're hoping to play at tournaments in the future (even local/small tournaments), chances are you may not enter unless you have a GameCube controller. So competitive play for this game pretty much requires that you're comfortable with the GameCube controller.
It is my belief that Nintendo wants to get back in the tournament scene with this game, which is why the adapter was made (any nostalgic attachment by fans to the GameCube controller is merely icing on the cake). Melee continues to be played at tournaments, Brawl less so. Now with a way to guarantee all players have equally fair chances (with no risk of wireless interference breaking someone's game), SSB for Wii U is most definitely the comeback to the tournament scene that Nintendo needs. I'd love to see 8-player Smash at tournaments too...!
Of course,
if you only play at home / with friends, then this is a non-issue.