Some of them aren't that bad.

Cloud's and Cait Sith's are based on HP. Yeah, that can suck.
Red XIII's and Cid's are based on MP. As long as you don't use any MP, you'll be fine.
Barrett's is based on the number of mastered materia equipped. That's great, since it can't change once you equip it.
Tifa's is based on the limit meter. That really sucks, since she'll lose all that power when she releases her limit.
Vincent's is based on how many enemies he's killed. Very good, since it can only go up.
Yuffie's is based on her target's level. Also good, since she'll usuall do quad-nine damage against really tough enemies. It's also what allows her to do really high damage when she's trying to morph her target.