If we place all four tanks side by side, we can see there's quite the disparity between them. According to fflogs, in terms of dps it usually goes GNB > PLD > DRK/WAR. Now for party utility, it goes PLD > GNB > DRK/WAR. Dark Knight and Warrior get outdone by Paladin and Gunbreaker in every way. They have the least dps and the least utility. DRK and WAR's dps does not justify their lack of utility. The two need a reasonable buff, be it potency or whatever helps them make up for what they lack.