Very few players can use the flamer safely. Most are as damaging to their own team as they are to the alien team, and most never know it as was previously mentioned. A flamer can drain 90% of the health away from a teamate and you will never know it and he will rarely say something because there was no TK.
I have disdain for most flamers and if they go left, I will go right and stay as far away as possible. But, as PIE said, there are uses for the flamer but it takes the right player to find them safely.
Diggs