It is acceptable because PLD doesn't rely as much on their healers to keep them alive as the other tanks. PLD requires the least healing as it has the highest mitigation AND effective healing received. Oh and enough supportive skills that can be powerful if utilized.
Guess what? EVERY class in the game relies on the rest of their group to "carry them to their clears"! WAR and DRK still need healers to heal them, and DPS still need a tank to keep the boss's arse exposed for rear and flank attacks. If everything was measured by how much DPS an individual could do, groups would end up with 7 DRGs and 1 healer.
PLD is the safest of the three tanks, it has the highest mitigation and is not lacking in ANY environment. PLD's mitigation is superb in physical and is great in magical damage. DRK is just better at magical but DRK is utter shit in physical. PLD brings so much utility that if a PLD knew how to use its utility healers will have PLENTY of time to DPS. As long as PLD has all that going for it, it should never, ever DPS as much as the other tanks. And PLD is hardly doing 8% less than a WAR going full DPS.
Also skill trees and different builds aren't "False Choices". They can be done in various ways where different builds CAN exist. You are probably looking at games that "failed" in their skill trees and cookie cutter builds that proved mathematically superior existed. But why not look at games that successfully made different builds possible?
I won't go into detail as it doesn't matter, but the point is some games succeeded at providing multiple builds for different classes.
WoW successfully had multiple builds per class, while some classes went down to cookie cutter builds per tree, some other classes had 2 or 3 viable builds per tree. I remember rogues having two subtlety builds and warriors having multiple arms and fury builds.
Aion for example had the Stigma stone builds. While every class had a couple of stigma skills that they couldn't live without, they still had 7 to 9 slots that they could build in freely. Defensive templars were as viable as offensive templars. Each brought something that had a place for. DPS clerics and defensive clerics also had their place. Sorcerers had buff builds that were viable and had burst builds that were also viable. Same with Spiritmasters' debuff builds and spirit builds.
The developers do not want to stray away from the fixed group setup of 2 tanks, 2 healers and 4 DPS, so hybrids are probably not gonna happen.
PLD already has enough mitigation and support to allow for healers to add more DPS. PLD is by far the easiest tank to heal as it receives the most effective healing.
Stop trying to play PLD like DRG. Your role is not to be the most DPS, it is to allow everyone to do their maximum DPS. You can help by providing more DPS and it was already proven that PLD is barely 6% lower than WAR (at ~940 vs ~1000). Not even much compared to the disparity between DRG (1300+) and BRD/MCH (~1100).