Ara, stop being a jerk.

So the problem with the posted idea is that it would be really easy to raise your def high enough that you barely take any damage. And since a lot of gear is shared between PLD and WAR this wouldnt really solve the PLD/WAR debate. Having an intrinsic difference in defense between WAR and PLD is probably an OK idea. Could be achieved by Job Traits.

But like Aratharn said, there are numerous topics discussing this very idea.