First I thought this might be a good idea, but after reading and thinking about it, I've come to conclude that this will only bring advantages to the Alliance, and disadvantages for the Horde.
Alliance gets to play horde and see all the strats and movements the horde does in BG, but not the other way around. I can also already anticipate having Alliance players sabotaging BGs on purpose for the Horde, either by doing wrong objectives (capping GY in alterac, going mine in arathi ect), trash talking all bg long, or just being a useless player cause they're facing their friends and people they know. How do you report such behaviour? Take a screenshot that will be disregarded because it not proof enough?
Alliance sabotaging BGs WILL HAPPEN, and some will be smarter then others about it.
If people change faction after this is implemented, it will be from Horde to Alliance. Why would an Alliance that already gets to play Horde change faction?
After this change, Blackrock people will create Humans/Night elf only. Why chose a faction when you can be both all while having the better racials?
What benefit does the Horde get out of this? Nothing at all, we will have enemies and spies in our ranks, same queues, same rewards, and a mass desertion on our faction.
This might balance the queue times, but it will make the Factions even more unbalanced then they already are.
For this to be fair for Hordes, Hordes should get more rewards (honor/gold, whatever), for playing Horde as a Horde character. Else the only reason to stay Horde is to play with the people you know on this faction (if they don't all faction change).
Or, cancel this idea and just make Free faction changes from Alliance to Horde. To me this makes a lot more sense then a one sided mercenary system.
As a Horde player, I vote no. Not because of the queue time, but because Horde is already the underdog faction, and this will only bring disadvantages to the Horde and benefits to the Alliance. THIS IS NOT A FAIR TRADE.