There are some oddballs among us (myself included) who like to imagine backstories and whatnot for our characters. I know for Isaac, the lore I've got imagined for him makes him incapable of using magic of any kind (to the point where I'd play GLD if it were still a viable tanking spec).
That, and there's something kind of immersion-ruining about having one character who is The One and Only Black Mage AND The One AND Only White Mage AND The One and Only Azure Dragoon AND The Superest Duperest Paladin Who Ever Paladinned, etc.