The first time I played through, I didn't use them at all. The second time, I figured I should get some use out of them. I found that they were kind of useless at first, but once you learn all of the abilities, they are useful, especially Yuna's. But, using these dress spheres takes away your ability to attack with more than one attack at a time, which is where the power in this game lies. So I've deducted that one should only use them when one is in a bind.