DRK is the last in raid environments except ones designed around their magic resist CDs. (Read: only half of Alexander, The Chrysalis trial if you're desperate)
Its only real advantage over the other 2 tanks is AOE damage, which considering it's a tank and not a DPS class is really only relevant in trash pulls.

PLD can do cool stuff as OT but with the DPS checks in Alexander/Alex Savage that's probably not ideal.
The way I see it, PLD is meant to be the plug 'n play tank. You've got your basic combos, some nice utilities, and good CDs, nothing too complicated.
I think its simplicity, or to put it negatively, boring-ness, is its calling card.