There was a video, I think by Mirai, where he was driving 2 healers from his tank using video fx (in ISBoxer). If you have ever multiboxed and tried to handle healing/tanking/dps simultaneously, that video will sell you on ISBoxer. It's a very efficient way to drive healing from the tank. For the multiboxers I played with, aggro management and movement were the two biggest issues for us (even in vanilla). Early on (I can't remember if it was in the initial release or not) the best multi-box tank was a paladin. That class was good at keeping aggro on several mobs at the same time. This becomes even more important if you're not going to bother with crowd control.