Keep in mind how the bosses for the raids work as well. The 3rd raid boss of Pandaemonium hits both tanks at the same time with every auto. It's very likely you're going to see people want the other tanks over DRK for that very reason, since the other tanks can keep up sustain and make it easier on the healers, whereas a DRK being there may very well make it much harder than it needs to be on the healers. I wouldn't take a DRK to that fight when I could take a GNB, WAR, or PLD instead.
