Samurai Job = DPS Job

[Sylve]

So... tell me again how Dark Knight is supposed to be a tank.

A Tank is one defined as holding the attention of hostile targets so the squishier Party Members can do their job.
The PLD Tanks with a Shield to mitigate damage.
The WAR tanks with his face and uses his anger to shrug off/ignore damage (Thats how i imagine Inner Beast XD)
The DRK could well utilize his sheer destructive force to terrorize his targets into dealing less damage. I could well imagine a DRK Sacrificing 10% his HP to fuel a spell which caused affected targets to deal 30% less damage to the DRK. Theres ways and means to make it work.