Also, I'd like to mention that sometimes someone else's offense can be good for your defense. For example, if you're low on health and you want to destract the enemy (except most bosses who ignore the others), target the enemy, then push TRIANGLE a few times to command Donald and Goofy (or whoever else might be in the party) to attack that enemy, giving you ample time to heal yourself.

I also advise using Tinker Bell whenever you can (against strong bosses...like Kurt Zisa). She doesn't cause the others to go away, and she'll stay during the entire battle (unless you tell her to go away). What's even better, she'll continue to heal your party, and she'll even protect Sora from death once.
By far, the best summon (but if they would've kept Bahamut...).