Mercs can choose at any time to join a Red or Blue teams. by 'Team Red' or 'Team Blue'

Players can be kicked by vote out of a team to become mercenaries. People can also be voted to be banned.
This inspires people to play politely (ie not as dicks) unless they are incredibly skillful (ie capable of surviving as 'killed by everyone' mercs)

It also allows small games to progress easily from FFA to a team game as players join.

Players who join a server and just feel like playing a dopey FFA can and the teamplay isn't disrupted. Team players expect Mercs to shoot everyone.