They’re going to have to add Mercenary mode when TBC Classic launches anyways, might as well start now 
And Alliance complaining about queuing with Horde, if Activision implemented it correctly, you wouldn’t have to. Example:
Consistent 100 Alliance and 1000 Horde queue for AV. 900 Horde goes hmm this queue sucks, I’m gonna go queue vs my own faction!
Now 100 Alliance queue into the 100 Horde that didn’t turn Mercenary mode on, and games are same Alliance v Horde as always, but the 900 Horde who turn it on get to actually play the game and not sit in queue. Don’t really see the down side here.