Unfortunately the world is full of petulant dicks who take themselves too seriously and throw a strop when their favourite toy is taken away - or they are accidentally killed by a teammate in Halo.
I'd hate to see these people in real life - what'd they do if you jostled their drink in a bar? Stab you in the gut? Get the management to chuck you out? Actually, probably not, because they wouldn't be able to do it at a click of a button and remain largely anonymous afterwards.
I have kicked out one person. That was during Firefight when this kid had systematically gone around and killed all of his own team mates until the other two players quit in frustration. Maybe he didn't understand the difference between Firefight and the rumble pit, but after the third sticky grenade to the face at point blank range I decided to kick him for betrayal before quitting myself. I know I could have just left the game and left him to do... whatever the hell it is he wanted to do, but I'd rather make a point to him that I considered his actions wrong, and I couldn't do it through speech because he clearly wasn't hearing any of our "What the fuck are you doing"s, or just ignoring them.
All this being said, I think there should be some watching-the-watchmen kind of situation making sure that people self-moderating aren't abusing their powers. Like questioning the umpire in tennis, you should only be able to kick people a certain number of times, which would force people to consider whether the offence was truly worth it.
I also think its a bit nonsensical to be able to kick people out when they are system linking with you. I did it by accident to my friend, and it made me wonder why I even had that option. I mean, I was in the kids house sitting next to him on an adjacent TV, how exactly was I going to get away with booting him from the game and continue on my own?
I just slapped him across the back of the head every other time he 'accidentally' ran me over...
All this being said, I've betrayed quite a few people by accident and also generally sucked as a team mate (I'm not playing this to a professional level, sorry people), but no one has booted me. Maybe I'm just hitting the Matchmaking at a time when more reasonable people are playing?