It depends on the game. Xenosaga 2 had way too many side missions and at some point, I couldn't even remember which ones I had open. So that felt pointless. However, with Kingdom Hearts 2 I couldn't rest till I've unlocked all the little things. Well, except for gummi ship stuff cause I didn't really get them.

Like the previous poster said, depending on the battle system I'd probably spend more time levelling than anything.