generally I would never quit in such a situation. BUT, a few games I have played in the past could often see you matched against opponents I could not hope to even compete against let alone win, I can usually accept that but when you get a string of them time after time I would rather quit early, I don't mind losing, but it isn't fun to know that no matter what you do you are going to lose and are just wasting your time. The exceptions to that tend to be some of the FPS's where even when you are getting stomped you can still have some individual fun.
Exactly, this is another view I've taken in over time, in that you can and will face an opponents here and there that you cannot hope to beat and never can. It becoming an easy win for them and a quick loss for the other player makes it a waste of time, since neither is getting the kind of match they want and it's already obvious that there is an imbalance.