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Speaking for myself here, whether or not I quit a game depends on the situation. Yeah, I've rage quit a few games before, I'd imagine everyone has. Normally I'll tough it out, like in the Blur beta. I may come in last in one race, but I know I have a shot at squeaking in a 2nd or 3rd place finish in the next one.
There have been a few times where I've been getting stomped so badly in a strategy/etc game that I've pretty much told the other person we were both wasting our time, and we'd be better off disconnecting and playing someone else as it wasn't fun for either of us.
In the case of Warhawk, I've played a few unranked matches online with some real-life friends where people who are obviously experts at it would connect to our game, get in the planes, and just light everyone's asses up from that point on. We'd all just disconnect at a certain point, seeing as it turned into a miserable experience for us and the five-star Generals (or whatever the ranking is) were obviously just game-trolling.
And if I am on a random team in an FPS game where everyone's great at it but I am sucking ass, I figure there's no harm in me leaving since I'm serving no other purpose than to bring them down (and sometimes they will loudly and repeatedly tell me this). If I'm playing with friends and sucking I don't really care, as I'm having fun goofing off and bullshitting with them anyway.
Botom line for me is that I play games for my own personal enjoyment, and I have a limited amount of time to play. While I will respect someone who has taken the time to set up a one-on-one game and tough it out, in games like Battlefield or Call of Duty I'll quit if I'm not having fun or feel like I'm wasting my time. There's always gonna be another piece of meat to fill the empty slot I leave.