As much as we'd like to, even a DRK can't kick a caster in the crotch repeatedly. A parry is required.
That was the thing though. On one side DRK had an immensely powerful and reliable bonus defensive. On the other, a less reliable but just as powerful one, and a bunch of bonus damage. DRK did fine in both magical and physical fights. It just did them differently.
And more broadly, it's not like PLD never had "its fights". Shelltron felt damn near as exploit in some.
