There will always be a superior choice when you take this approach. You can try to create a scenario where different advantages help in leading to the clear in different ways, but once people figure out the best way to do it, that will become the standard. That's just an aspect of human nature that is part of the landscape in ANYTHING that involves more than one player.
And why are we going back to demonizing performance parity? Especially when the lack of performance parity is one of the main culprits behind the problems we're currently seeing (encounter design being another)?
Class identity can still come from mechanics, abilities and aesthetics while still remaining equal to other members of the roster in performance. Are you really that bothered by WAR and DRK dealing the same damage as a PLD?It is simply another avenue for providing balance, and one that SE has actually admitted their faults in with this last tier. True balance will likely still take a bit of tuning on the job side as well. But to design content that actually makes use of their differences, as in entirely unique strengths rather than just difference in strengths in common among all three, increases the ways we have to retain tank identity without leaving one or more behind.
Which is not a bad thing. The utility PLD was given is not the type that is or should be needed all the time. This is why I facepalm at the people who complain about Clemency not being a mandatory ability in the rotation (it's situational off-healing, emphasis on situational) and those that expect Divine Veil to be up all the time, every time (because expecting to do so would be like expecting this or this to be used every time it's up instead of during oh-shit moments).But can we really say that even if PLD is given as much MT (assuming Grit/Def/Shield) dps as DRK or WAR, and if it somehow gained DRK or WAR-level AoE, it would be taken? It would still ultimately be burdened with abilities that see little to no use in most situations.