I really like both Firelance's and Drakona's analysis.
I disagree with Drakona, still, although I think it's a very smart response. Any party would be less well off without a marginal member.
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. If the monster is not incented to attack the tank, then the tank becomes a sub-par striker. That is, by the way, generally what happens (although I do agree that halting movement and movement / terrain escapes do alter this significantly).
Attacking the tank is a one-way trip to losing for the monster -- as any party knows. Gonna attack the brute, the soldier, the leader, or the controller? You know what you'll do.
Drakona's point about raising the incentives sufficiently is well-taken: that is, perhaps this is solely a point about insufficient incentives to attack the tank. And I think that I could leave it at that point, agreeing with Drakona, if it weren't for the underlying logical problem that I initially discussed: the defender role relies on a less-than-marginal contribution to the party.
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.
best,
Carpe