Well early in the game, you kinda have to use everyone because of their particular strengths against certain enemies. Once you start to seriously cover the sphere grid, though, some characters ccan be phased out. But on the topic at hand, other than a few Yuna-mandated fights and a Kimhari Mt. Gozel (was that the name of that place?) boss fight, you can use whatever. I never bothered with Riku, she sucked. Tidus, Auron, and Wakka/Lulu ftw.
