If you don't use fighters that have six buttons, why would you keep five? I can't think of a lot of five button games off the top of my head.
I agree. There are some stragglers out there, but to me it's either 3 buttons or 6 buttons. With 3 buttons you can play all the old classics and the rare game that has a jump/fire/grenade type feature and none of the fighters.
I guess overall it depends on who your building for. I went with 6 buttons because of all the people wanting to play the thing. I'd rather not have to deal with the look of disappointment when they find out they can't play one of their favorites.
As for the trackball... yeah, I wish I had room for one of them. Not so much for the mouse, but for centipede, crystal castles and missle command. When Mamewah is setup properly - you rarely need any other input then your cp. I had to make a choice and I went for the fighters instead.