The grinding situation is awful funny in CoM. If you take on all comers, you won't need to grind, probably, but you will be
bored because as Gram and 1ofamillion said, it's just one room after another. God help you if you try to clear out the entire floor, like I did in my first playthrough.
(even with Riku and the terrible GBA Atlantica deck. *shudder*)
This is a game I
want to rush through even though I enjoy it very much (in fact, it feels like rushing through it would be an accomplishment!), but I never do, because I know how much trouble I'll be in. Sometimes I think: it could be okay, because with the right knowledge, you can win as Sora without grinding! But with Riku? That's a fine line. I remember reading some experienced "Zero EXP" KH players trying to get through this game while levelling as little as possible and eventually one of them threw up their hands and admitted that Riku was only doing 1 damage without Attack Point upgrades and they just couldn't
take it any more! But if you raise Riku's level, especially with grinding I'd imagine? Breeze.
As for EXP Orbs: until I started to see them in Shooter games, where you can only take one hit and moving out to grab something could kill you, I never understood what EXP Orbs add to the game. They still don't make sense in the GBA version, the PS2 version, they don't make sense in Fable... so strange.