The issue with with the OPs suggestion is this - what happens if you extremely unbalance the fight because 1 group has full team, while the other 2 have significantly less people? Also, just because a person queues doesn't mean they'll be ready if the queue takes longer than a few minutes. They may be forced to quickly afk to take care of something and don't have the time to unqueue. Everyone is guilty of this happening to them, and to force queue them into a pvp fight where now you have someone afking and unbalancing the team is just unfair to everyone. Sure you get to play, but you'd likely have no chance of winning if this happens.