Not to put down the OP or anything but this kind of thinking is part of what contributes to the homogenous nature of the game. The simple solution to low MDEF is simply to have your healer SS + Adlo you before something like Megaflare.

Why are we trying to make all the classes the same? I like the idea of having to look out for certain things when you bring a specific class. Something better more than just the difference between ranged/melee e.g. Ifrit Ex, Garuda Ex.

Besides, outside of endgame DRGs get a lot of bonuses from their high Def and HP. I guess the nature of the beast is that everyone will focuses exclusively on raid content when discussing classes, but taken as a whole there's no reason they need a buff. Then they would actually just turn into little mini-tanks.