The game is meant to be playable/completeable (is that a word? it is now) with a full party of 4 regardless of who is in that combination. It's perfectly viable to only recruit the ones you intend to use. There are some situations and quests where there's a definite advantage to having spesific people with you though. It should also be possible to finish the game solo, they talked a bit about it in the last part of the Panel from Hell where they had Shadowheart only, though that is naturally harder.
