A lot of the replies on this thread ignore OP's entire point. The problem is not that PLD can't keep hate. The problem is that it doesn't matter who has hate (among frontline jobs, of course). The tough mobs in the game AoE like all hell and hit every melee regardless of who has it's attention. Having a PLD doesn't help the party stay alive... it slows down the killing process.
I love my PLD and wish it was more widely accepted (I don't have all the Relic/Empy toys). Adjusting the enmity cap for PLD is a good place to start (if for no other reason - make the tank appear to be tanking). What we really need, however, are mobs that readily wipe unprotected melees, and PLDs who can effectively protect them.
I like the idea of a -PDT sphere-effect. I also think Cover should be a job trait (with maybe a 90% activation rate).