From my experience I see DRK as the farming tank. Of the three tank classes they have the highest DPS while in tanking stance. They have acceptable levels of mitigation (not quite as high as pld). They don't have the utility that PLD or WAR offers but their increased DPS can help with dps checks if that is your problem area.
Alternatively some could argue that the decreased defense will cause healers to have to heal more and thus less dps uptime for your healers. Either way DRK do not make good OT but can be acceptable in MT roll.