"Most people on PvP realms like having a faction advantage and choose their server or move to servers specifically for that reason, which makes evening it out fairly difficult. There also tends to be a faction imbalance on PvP vs. PvE realms, where PvP realms tend to be Horde heavy and PvE tend to be Alliance heavy. So when we connect a few low-pop PvP realms together it's somewhat likely to only be able to be matching imbalanced realms with imbalanced realms. Considering the desire for a faction advantage, even if we did achieve a balance I'm honestly not sure how long it would last. In any case, with Connected Realms both sides will be getting more players, and bringing up the amount of people on each realm is the goal."
Blizzard seems off with the subscriber's thinking on joining a pvp realm. Most people just want to join the pvp side that isn't dead, not because they want to have the faction advantage. I'm pretty confident that if they were to poll the subscriber base, that an overwhelming majority would want balanced realms.
They are really dropping the ball on this technology when it could be utilized to revive world pvp/activities.