I do it constantly on Xbox live, however, I throw in the twist of trying to make it as hilarious as possible, and usually I fail spectacularly at whatever game I'm playing. I especially do it on games like COD, to make fun of the people who are trying to talk smack and be serious about it. I'll also use the crappiest weapons I can find for extra hilarity. I'm probably the only person in COD history to get a 15 kill streak with the Javelin while yelling nonsensical Caboose quotes.
For example, if I was playing Left 4 Dead, I may charge the tank with a guitar and scream "I AM MRMOUSTAFFA!!! I WILL EAT YOUR BABIES AND CRAP OUT THEIR SOULS!!!!" only to get punted across the level and die. I try to do it in a clearly ironic matter, so people know I'm joking. I've found its its a great way to put some levity in a situation, and it deffinitely prevents people from yelling hacks, even if you're dominating the match.
Long story short, I do it to have fun and mess with people. I know people who do it being serious, and cant stand them, and I hope I dont come across that way to others when I'm online