Well in order to complete the game it is required that you use each party member at least once. My main party consisted of Zidane, Vivi, Freya and Steiner. That was also the party I used for the final boss. The rest I hardly ever use, unless I really need to.

Zidane and Steiner are mainly for attacking, Vivi for Magic and Freya for Reis's Wind and Jump. Usually I don't bother having Healers in the party.