Umm... What?
Say a mob can take 450 damage before dying (to make it easier)
Fighting 6 mobs one by one, but doing 450 damage per rotation, it means 6 mobs take 6 rotations to die.
Fighting 6 mobs together, but doing 150 AoE damage per rotation, it means 6 mobs take 3 rotations to die (3x150, applied to all mobs).
So yes, the mobs die faster individually, but the end result is still that the 6 mobs are dead much faster if you AoE them down. If you increase the numbers and have 9 mobs in the same group, they still take 3 rotations to die, since all enemies take the same damage from one attack. One by one, they'd take 9 rotations to die.
(Note: I'm using "damage per rotation" instead of "damage per second", since it just looked silly saying 6 mobs took 6 seconds to die :3 And since it's really late, I don't trust myself to go for higher numbers and get the math accurate ^^; )
But yes, it does mean that some of the mobs will damage the tank for longer. Which is why you have defensive cooldowns. When I pull three mobs, I rarely have to bother with defensive cooldowns, because I don't take much damage from them (expert roulette dungeons exempt, since those mobs do hit harder :3 ). If the tank isn't geared for a speed run, or the healer isn't geared to heal a speed run -- or the dps aren't geared enough to support a speed run by killing things quickly enough -- then a speed run will not work. You wipe, and get to start over again.
Personally I prefer killing things group by group, but if I have a healer I trust and if I have at least one (well-geared) AoE-heavy class, I can pull two or three groups even in dungeons I'm not that familiar with. The only places I've ever pulled more than that is Brayflox HM and Wanderer's Palace. Mainly because I'm familiar enough with those two places to know how much I can take. And that's often because I've had healers or dps who've told me to "pull more". If I see the "someone is new to this place", I never do it. And especially in WP, I do tend to take the time while waiting for the dungeon to start to check out the gear of the rest. If one or more people are still in i70 gear or below, I'll limit pulls to two or three groups at the most; even with a non-overgeared healer, that's how much I know I can handle -- heck, I've had a scholar on lazymode (aka /follow on someone in the group) in there, with just Eos keeping me alive through that XD
As for the "pull mobs into boss rooms", I suspect those would be either troll tanks or failed lockouts (where you pull mobs to just outside the boss room, tank stays just on the inside and someone pulls the boss over -- once the area is locked, the mobs de-aggro and go back to their spots).
Yes, some tanks do seem to like biting off more than they can chew, but for at least some of those I actually suspect it's because they've always been told by DFed people to "pull more!" (and when they don't, been called 'crap tank' and people drop out) and then think that's what you're supposed to do if you want to be a good tank, instead of realizing it's more the opposite way around. That was one of the reasons I stopped DFing dungeons for a long while, back when I was still learning, and only went with friends.
Except he was talking about Wanderer's Palace and upwards.... which means lvl50 dungeons o.o