1 PLD setup not only makes group compositions less dynamic where its either PLD or WAR and DRK/WAR and WAR/DRK and DRK(lol).

It would make content designing alot more constricted in that they have to make all content manageable by 1 tank. I.E Sephirot's two towers. PLD can not stand in both.
A1S, PLD cannot tank both bosses together. A4S, a DPS cannot tank the add in the room. And a plethora of other mechanics where there needs to be a tank in 2 separate locations.

If you make raid content where only 1 tank is ever needed, why would you honestly ever want 2 tanks. The other tank is literally doing nothing but DPSing in that situation. Why find 2 tanks when you can just get 1 PLD? What if you only have a WAR/DRK? Now you HAVE to find another WAR/DRK or eat a the penalty of having lower DPS with a PLD/Tank setup.