What about creating an ACTUAL Dark Knight?

[MaraD_]

Attacks that cost HP or that do more damage at lower HP may be "high risk, high reward," but the risk and the resource expenditure is shifted from the player to their healer. In a single player game where you control all of your party members, this is fine, because you have control of both sides of this. In an MMO setting, though, it doesn't work as a job theme. You'd end up deliberately throwing yourself into harm's way for more dps while your healer picks up after you.

The one area that you can work on is the lifesteal angle, as per Final Fantasy Tactics.

I agree with this, but just to clarify, thats not quite what they meant.
But I do agree this style isnt the best thing to do in an MMO, and not just for the reasons you mentioned.
(Thinks on it, actually, I think I just came up with a work around, to make that work. *goes to write that down for personal use*)