Precisely. I feel as though some people have not used PLD in a difficult long lasting fight. Many of the "advantages" people are feeling PLD has really wane as a fight drags on. I personally have no enmity issues on either PLD or GLA, which renders Cover useless and Holy Succor less effective. The fact is, most of your healing should be on yourself, as you should be taking the majority of the damage. Let the WHM decide if its worth sparing MP to cure others after AOE's, or let them heal themselves (through abilities or waiting a few seconds before doing anything else).
In shorter fights, the difference between GLA and PLD is not as noticeable, its when you do these longer drawn out boss fights that it becomes truly apparent. I would bet anyone claiming PLD is better than GLA is just experiencing a side-effect of having a WHM and possibly a BRD as opposed to CNJ.
EDIT:
I urge you to use Hallowed Ground as an "oh-shi!" ability on Ifrit. The result could very well be a dead PLD. An instant heal and an evade (to help prevent interrupt) + cure heal HP which work for both physical and magical attacks, and stalls for long enough for a WHM to get that critical cure in.
Also, Holy Succor does have a short recast, but also a massive MP cost, and relatively long casting time for someone getting hit. While you suggest this spike enmity is nice and could be right, we have no testing to prove this theory. I would suggest, however, that the more cures over time you can perform on GLA will outweigh the enmity gained by succor. Enmity at this point isn't even an issue anyway.
You suggest a THM using sanguine rite, though now instead of the PLD having to be on class instead of job, its the BLM. BRD works too but again this just puts more reliance on BRD, and both of these can still be done to a GLA rather than a PLD.
The ability to make up for the drawbacks of PLD through other party members does not make up for it's losses. You're comparing apples to oranges being assisted by bananas. You fail to realize that those same bananas could assist the apples rather than the oranges.