Having run MgT several times now, I've never had a problem on Vex, heroic or regular. Prot warrior, heroic geared, 0 arcane resists. If your healer is good, there's absolutely no reason to have your dps go and steal the adds, the tank can easily handle the debuff, and the additional damage means you won't lose agro. It stacks up quite nicely, and your healer doesn't have the headache of dealing with dps dying mid-fight. Try it some time, you won't be disappointed!