Actually, I find that having any more than three jobs for your characters can totally offset the balance of the party. Quite literally, I have three characters set with certain gambits, and then the other ones are just absolute clones of those three. I chose two characters who had about the same stats and made them the closest I could come to three types of jobs that I deemed necessary. Vann and Basch are my Warrior/Knight/All-around smacker. Balthier and Fran are my ranged attackers, and Penelo and Ashe are like my white mages. Then for the Black Magic, I just split it three ways so that everyone wields two or three elemental spells. Also, all of my buffs (protect, shell, etc.) are spread out between the three characters.

But, basically, Vann and Basch are geared for attacking, Fran and Balthier are geared for casting some support magic and attacking flying creatures, and Ashe and Penelo are there to heal my characters. Anything that is not a part of my gambit system is generally used so infrequently that I don't really mind having to manually select it. So, basically, once one character of a particular job class is one level higher than the other one, I switch the other one in so that everyone levels up evenly and so that changing character right in the middle of a battle will disrupt the flow of my party as minimally as possible. And, just as a side note... YOU'RE CRAZY!!