I think, after going through some work to get all of the abilities for it, the Calculator is one of the best jobs in the game. Their 'Math Skill' ability, when used correctly, can hit almost anyone on the field, anywhere on the field. Also, when using math, they don't use any MP for their spells! They're pretty weak, and slow, but if you master the class, then switch to, say, a Summoner, you could be using powerful spells for free! Plus, you could use Summon Magic, which the Calculator can't do.
Most useful FFT job class(es)?
Calculator is useful, but when magic fails me I turn to my Samurai. Good attack skills that don't hit allies and vice versa on support skills.