Another obvious point I forgot to mention. With friendly fire off, it's a very simple system - play the game right and you'll be fine. Accidentally run over a teammate and you'll usually be good unless you've done considerable damage to their invisible "health" in the past, in which case the prompt for booting's more likely. Contrary to popular belief this is an effective system and can probably be deemed useful about 90% of the time, the other 10% being when a griefer just weakens you rather than kills you or the game pulls the old "it let him kill me five times and I couldn't boot him but killing him once got me booted!" problem.
This is not the case at all with Friendly Fire enabled. Now your team's troll doesn't have to worry about any repercussions, other than the fact that he isn't doing any literal damage. Rather than having an asshole steal your Warthog and splatter three teammates and get booted, now you have a guy running around, knocking stuff over, pushing teammates way out of the way or even off the edge, and who can stop him? Not any of your teammates, that's for sure, because now an invincible griefer is stealing some of your power weapons and there's nothing you can do about it.