Maybe I'm playing wrong, but I actually find Drk to be better than PLD.
As a PLD, I run around stun locking people and throwing up debuffs with full swing. Skill speed/attack power are WAY too low to do much dmg, so, I usually just eat their stoneskin with spirits within and let the angry mob that's following me do the rest.
As Drk I feel like I can actually be more help. Drop salted earth on nodes. Pop Blood weapon when dealing dmg to lone targets. Pop blood price when charging a big group (for distraction purposes). Remember to use Dark arts whenever Mp is high. As long as I remember to disengage after each dip into the fray and keep Blood wep/price up during siphon blade assaults, I almost never run out of Mp in a match. Being able to close distance with plunge and follow it up with a stun is awesome too. I usually average around 2-3 kills and maybe 1 death per match... Dunno if that's good or bad for a tank.