Real tanks - responsible tanks, in a PUG will look at the dungeon content, know it's hazards, examine and test their healer in the opening pull, and weigh up their competence and risks BEFORE they decide what strength/vit ratio they're going to roll with and the size of the pulls they might attempt. Tanking is a stressful enough job without immediately getting the ire of your party right out the gate by screwing things up by pushing your team beyond their limits.
Tell me how many of these you run into in the DF, compared to the ones that just go gung-ho without caring. I'll wait.