Rage Quit?
Never out of "anger". I've quit "mid-session" quite alot in a number of games, usually the ones which involve time investment.
Most notably, Cyanide's interpretation of Blood Bowl. The only way to reliably get a game (outside of a league)is to use the Ranked Match Maker system with all the turn limits set from 40sec all the way to 4minutes (for each player turn - when you play the game you realise that a 4min turn can take anywhere up to 15mins as the clock stops when your players are moving).
On numerous occassions I've quit mid-game (conceeding the match, giving the opponent an effective X to X+2 TouchDown victory, along with my earnings and the MVP award) during a 4minute game, normally against Dwarves. Not out of "Oh My God RAAAAAGE!" *disconnect*. But out of desperation of sitting staring at the player fumble around for ~15minutes a turn not really making any ground, only for me to make my moves in ~45secs and pass it back to him to fumble around for another extended period of time. I'd rather take the loss, and find someone to actually *play again* instead of reading the book that I keep on my desk to pass the time with.
In short? I'll "ragequit" (rather loose definition of "ragequitting" but I'll run with it meaning any "quit" before the end of the match/game) if I see that my time invested far outweighs the enjoyment I gain from the game. Time is precious, and I'd rather lose and enjoy it (in a reasonable time) than win really, really slowly and in a boring manner.