It's quite simple. PLD is supposed to be the best defensive job in the game and it takes the best gear in the game to be better than a blink tank for staying alive. So square needs to give PLD better spells/gear/abilities/traits for mitigating damage. They're so worried about overpowering it that they make it useless.

On another note, PLD could really use it's own provoke so people can double down /war or still hold hate /dnc.

Between those two things, that should fix PLD.