There is a benefit for staying in the game past the point where you know loss is imminent: Favor points. Depending on how badly you lost, this might be as low as 15 (particpant award), but it is entirely possibly that you may pick up other awards as well, such as kills, deaths, citadel upgrades, damage, and so on. Often this means even a lost match can net you 40 favour or so. It isn't much mind you, and given the choice, I would certainly prefer a 'forfeit' button to obtaining a small amount of favour, but there is still at least some small reason to stick around for the loss.
Additionally, comebacks are entirely possible. I have had at least 3 games in which the winner was virtually decided, and had the game turn completely around for the other team. One of these games even had a teammate leave with a 'good game' comment, that simply lost him the award for the win we eventually obtained.
To Agent of Kharma.. I am incredibly surprised you would post that you quit for such a reason, as this appears to be the very definition of 'rage-quitting' To explain.. there are no favour items that would push the other player to a point that they would dominate you to such a degree given all other factors being equal, and from the way the game it designed, it's difficult to imagine that an 'exploit' would exist that could alter his damage (otherwise we would see characters inflicting 9999 everywhere). It seems more likely that either A: There was another player supporting him (regulus) or his team obtained the +15 health regen flag or the +10% damage flag, or both. Additionally he may have simply gotten 'lucky', either in direct hammer hits, or through autodamage which is already known to be on a randomized basis, or perhaps used combat potions and you lagged out of the animation.
What bothers me the most about trying to justify leaving in a huff, is that the leavers appear to be under the impression that their opponents are pricks that want nothing better than to laugh at their failure, when a simple comment to 'all' (using shift-enter) asking how your opponent accomplished that win, or a "You slimy bastard, I'll get you next time!" might lend to starting a friendly conversation out of what could otherwise turn into a silent festival of frustration.