I think its fair that PLD should have the weakest damage output from all the Tanks.

First of all, your competitors swing oversized axes, oversized swords and a sword that does explosions. Your regular pokestick isn't much in comparison.
Second, PLD should have the edge when it comes to utility, at least in theory. If Squeenix would finally manage it to remove all the utility bloat from DRK/WAR/GNB (by replacing them with something more fitting for the Job that doesn't contribute to homogenisation), then even the low dmg output would hardly matter, because PLD has other qualities besides that.

Tl;dr: it isn't always about the dmg.