This is pretty much the bane of some of the later FFs, whereby to encourage customisation, they had to basically remove the central character's roles. It was even worse in FF8, whereby the only real different between characters was limit breaks (barring of course Squall's 255% hit rating). FF9 went back to a more strict role system, and FF10 was kinda like that, but by the end everyone again could be identical with the exception of limits.