1: Most of the jobs in this game come from the jobs in FFIII. The rest can be found in FFIV~V.
2: WAR was never a tank other than in FFXI. In III it could wear heavy armor, but it's ability was to deal more damage at the loss of defense. Actually, WAR completely follows it's lore as being able to debuff with it's weapon too (As seen in FFX, FFX-2)
3: Get your head out of the DD clouds, what you want for the job is to bring it closer to WAR rather than it's own job. You might as well blend them together.
Seriously though, if all you want is Attack bonuses and DPS bonuses why are you playing the job? WAR does all of that better in every way. The only time DRK stands a chance is outside abyssea when there is no BRD.
ANSWER THIS: There has to be some discrepancy between DDs or else why even play DRK? In terms of DD it loses to WAR and SAM and DRG in every way.
