DRK and WAR both provide better party support than PLD right now.

DRK brings the Delirium which in turn lets you drop MNK for NIN. So, because of that, you gain an overall increase in raid DPS and NIN utility. That's a bigger deal than anything PLD brings to the table.

WAR brings Storm's Path and Storm's Eye. Again, two party support abilities that crap all over PLD utility.

I think most PLDs would also be fine without a DPS increase if you gave them real party support rather than situational niche abilities and defense that actually matters -- so enough defense to actually allow healers to DPS more to make up for the tank DPS loss.