I didn't take offense, no worries. At least your criticisim was intelligent and thought out. Unlike "go level war" lulz.
Your comparison to other drks in the ff universe is exactly the problem, there is none. DRKs from a game like FF:Tactics can be some of the most deadly units in the game. Either the dev team doesn't know what they are doing, , or drk doesn't translate well to an mmo.
Let's look at our job defining ability Souleater and compare it to a few other versions.
FFIII (DS version) - Damages all enemies at the cost of 20% HP.
Damage inflicted = weapon atk x 5 +job level x10 + current hp x 0.9
FFIV - In the DS Version, damages a single enemy at the cost of 10% HP. Attack power doubles and is considered a status that lasts 3 turns. Attacks are also considered dark element. The damage formula for Dark Wave in non-DS versions are as follows.
Damage = Attack multiplier * Attack Power * 1/2
Damage = Damage + ((Damage * random(100,150)/100) % 256)
HP loss = Current HP * 1/8
FFVIII - Damage will be tripled and the user will take damage equal to 1/10th of their max HP.
FFXI - Adds 10% (Or 12% with gear) of the player's HP to their next physical attack.
As you can see, as far as I'm aware, FFXI souleater is the ONLY one that bases your damage entirely on amount of HP you have. Bad idea if you ask me.