A Ninja can pitch in with Haste if you don't want to spend two or more turns having your Black Mage cast it. As for Thieves, in the NES version, they prettymuch never fail at running from battle.

Ultimately, a second Fighter/Knight would probably be more useful in battle, but then you have to worry about generating the gil necessary to buy a second set of all those weapons and armors. And with regard to aesthetics, having two of the same sprite in one party is unsightly.