Two things.

First. Anyone that exclusively multiboxes any game is going to be bad at that game. I can't play a shaman or a paladin nearly as well as someone in a high rated team, or a high end raiding guild. Because I don't play a shaman or a paladin. I multibox. I take the abilities of several classes and combine them into a single unit. In a way, boxing is its own class. Ask the best mage in the world to login and play a class he's never played before and you'll see the same result.

Second. I don't think that multiboxing and multitasking are synonymous. In fact, you could say that multiboxing successfully is an attempt to do as little multitasking as possible.