When I've been playing and someone screams "TEAMS!" it usually balances pretty fast. I dunno, from what I've seen it's fairly well self-policed.
I also agree it has to do with playstyle. The first 30 minutes or so when I'm playing around with the light and medium 'mechs, my kd ratio gets pretty high because I will take the time to go back and repair. Once I break into the Mad Cat, I will die most of the time without repairing once, and my kd suffers.
Once, I was pretty happy with a score of 13-0, not so much b/c I had a great kd ratio, but because I had managed to make that many kills with Raven and a Puma without dying(I lost 2 Ravens and ran back to base to buy a Puma). One of the most exciting things for me in this game is to do hit and runs on vulnerable enemies. I measure my playing skill on how long I can get away with this without dying. It gets REALLY interesting when the heavy 'mechs start rolling out.