Okay, first of all, 'tank' is not an MMO-specific term. Maybe MMOs mean something extra technical by it, but I'm pretty sure it's an ancient, general gaming term of art. I mean, I use it to describe my actions in games as diverse as Starcraft, Nethack, and Left 4 Dead, and I've never even
played an MMO. It just means 'the guy who saves everybody else hits by taking them himself', which is a pretty broadly applicable strategy.
Anyway, I wouldn't take it as being synonomous with the defender's role in D&D. The 4E roles are kind of like general packages of more specific roles. 'Tank' covers the 'drawing fire' part of what defenders do, but they also have 'debuffer' and 'meat shield' aspects, along with whatever you'd call obstructing movement and general tactical positioning of everyone. And leaders are part 'buffer' and part 'healer' and have other aspects. The roles aren't sacrosanct for the purposes of discussion. If you want to talk about healers or tanks, talk about healers or tanks. That's legit.
Carpe DM said:
The correct comparison isn't between having a tank and not having a tank, but between having a tank and having another, for example, striker.
Pardon me, sir, but I think you are changing the question. You didn't ask whether tanks are better than strikers, but whether they are self-defeating. Perhaps an all-striker party is the bomb--I've heard that it is--but that's beside the point. Even if strikers were always better, that would make tanks merely suboptimal, not paradoxical or self-defeating. Tanks do contribute exactly what they say they will: they defend the party by drawing fire. Whether they'd be more helpful by doing damage instead is an entirely separate question.
Carpe DM said:
When the defender performs her role, the rational response of everyone in the party ought to be "darn, I wish that hadn't happened." That is not the case with a well-placed fireball, a knife in the back, an arrow to the head, a healing spell, or any other core class function.
I don't think that's true. When the
monster acts, the rational response of everyone is "darn, I wish that hadn't happened", because monsters exist to oppose the party. But remember that the monster's hit is not when the defender acts. The
mark is when the defender acts, and everyone
does cheer. The defender limits the monster's options, which is what he's there to do. When that results in him tanking, that's still a win for the party, and they should still cheer.
Put another way, from a DM's perspective, my players often her me sputter, "Okay, the monster is going to charge over and attack the rog . . . er, wait, you marked it last round, didn't you? . . . okay, he's going to . . . er, I think he has an ability that . . . screw it, he attacks the fighter."
And the players smile, and they should smile. The defender worked.