Think there would be some implementation challenges.
You'd need to be able to group up with opposing faction, you'd need to be able to communicate with them too. Since you could group up with opposing faction, you could abuse/exploit/take benefit of the implementation. E.g. boosting your Alliance alt with your Horde main. Using your Horde healer to heal your Alliance PvP character, while engaging Horde.
I suppose mercenary mode and RDF works as they're in instanced scenarios, and use a system to collect the pool of players.