Ok, I'll take the bait.
It was unbalanced because majority of PVP population picked alliance, horde had barely any pvp guilds, arenas were like top 8 Alliance, 1 Horde, 6 more Alliance and so on. So anyone wanting to PVP picked, guess what, Alliance.
Now, if you can get this through your thick skull, out of 12k online, say 2k are leveling, 5k are PVE, 5k are PVP. Out of those 5k almost 4k are Alliance. Making Alliance queue times insane. So now you have players just sitting there waiting, and other players just getting instant queue pops and getting obliterated bg after bg after bg (repeat).
Alliance are sick of waiting 5 minutes and having no challenge.
Horde are sick of getting stomped.
Introduce Mercenary.
Now, if you're Alliance, you're bored of steamrolling through unmotivated players, you don't feel like waiting another 10 minutes for the next round so you turn on Mercenary. Now there's no long wait time, it's actually challenging because other team is also PVP geared, they show some resistance.
Only issue here is that you're a little ***** who doesn't like the challenge of playing vs people who actually know what they're doing.
So keep your statistic of 1 player to yourself. Everyone here seems to be ok with it, but oh we've got a prince here, let's **** over the rest of the players to please one.