I really can't believe how many people have suggested this, and how many mods it has taken to reject them.
Pros-
-would allow two teams, or 3 1v1s
-would stop 2-teams in dm from winning all the time
-could make boosting less common
Cons-
-allows 3 teams instead
-if there are two teams and a single player, that player basically has to take out one team and immediately take out another team and sometimes a single player in dm
-if there is a 1v1v1v1, it will be MORE indecisive than a 3-player ffa
-more cleanups
-even shorter cames
-makes refill practically impossible to occur
-causes people to have less gear going into dm
-bigger dm arenas for every map, or a feeling of clausterphobic chaos
-the original Hunger Games WAS a 2v1
Those were just the ones I could think of.