Strong disagree. I think all tanks should have some specific "sub role", with PLD being a great support tank (can shield main tank and assist in heals if needed among other things). I would even say give PLD a sort of Raise spell with long cooldown.

I would rather the other tanks (and really all jobs of the same niche) to have MORE variation in the sub things they can do. DRK should have more DoTs or (actually useful) debuffs. Maybe a vampiric healing sort of buff it can give. WAR should have more DPS options or perhaps more spike damage abilities.

Point being, all jobs should have that sweetish balance in their main purpose, but there should be more variation below that.