In previous seasons, for 2's and 3's, there was not enough burst to make multiboxing viable.
Basically having two or three people control the toons gave them enough simultaneous actions to win pretty much all the time.
Currently there is too much burst, so a team might do okay in these brackets, but if they reduce burst this might not remain true.

I doubt anyone could control a DK/Pally, a priest, and 3 shammies in the 5's bracket, mainly because there are too many things to click for each class.
But, if you were to play 3 shammies and have two friends, one each on the DK and priest, that might make a very viable team.