I've only had one experience with a Dbag tank when a friend and I were doing Darkhold runs for swag.

Darkhold is NOT a hard dungeon. My buddy who was running BRD has PLD tanked it for our LS manymanymanymany times. On his BRD he likes working in Flaming Arrow to his rotation, and he was waking mobs the BLM had slept (I was healing as SCH). He knew he was waking the mobs, the BLM didn't mind and just stopped sleeping them (he was new to the run so we were going for his clear as well). Everyone was nice, except the PLD. He started the resulting conversation with "You're a special kind of stupidass" directed at my friend and it just all went downhill from there.

Again he was talking to a PLD who'd tanked the place three or four dozen times. You don't remotely need a sleeper in Darkhold. The PLD was wearing AF gear with a bunch of level 25-40 ACCs.

Oh my lord the attitude this guy plopped out. You'd think we had sodomized his mother. It quickly became apparent why he was mad though. Even after my bro quit using his AOE the BLM didn't bother sleeping mobs for a while and the PLD kept dying. The guy couldn't pull - he'd make a 3 mob pull and end up with half the room on him and we wiped. Then he went off on the newguy BLM and berated him so bad he left the instance. As that was going on I told him "if you're so unhappy, leave." The reply was "lol its good 2 be the tank!"

My point with relating that story is that I think the asshats are drawn to the tank role for a specific reason. They want to be an "important, needed" job so other people can't just tell them to shove it, which means they want to avoid being one of the dime-a-dozen DD jobs. But they're also too selfish and egotistical to take a backseat role as a squishy stick-waving healer - the other critical, high demand job class. So bam, they pick a tank. Now they get to look cool on the front line, get tons of invites, and can force other people to put up with them.