As others have said, the split is there probably so they can use mechanics like counterattacks and such without having all of the tanks fight for aggro for the DPS gain. If they're going to commit to the split, I hope they'll go further in the idea of taking damage to deal damage; something like a reverse Wildfire where the more hits you take, the higher the potency of the attack. Vice versa with the OTs and the way they play. I think if they nail this, it could do a lot to break up the years of stagnation that we've seen with tank design.