DRK/PLD is moreover a tossup, since DRK loses much more options from not-tanking than PLD. A DRK loses Bloodprice, Reprisal, Lowblow procs, while PLD only loses their shieldswipe. So from an theoritical optimal standpoint, it's more raiddps to have the DRK MT and the PLD OT in that sense.
However I agree that PLD's enmity combo in their dps combo makes it hard for a DRK MT to maximize their DPS when they have an OT-PLD as their co-tank, so from a practical standpoint, it's sometimes difficult.