tfra11:I would say also that there is a difference between quitting and acknowledging the inevitable.  In backgammon in particular, I will resign only when I have no mathematical possibility of winning.  To resign earlier would be quitting.

I would go so far as to say that in other games, such as chess, you should resign a clearly lost position because to play on implies you don't think your opponent has the ability to play out the clear win.  Of course, the question whether a game is "clearly lost" requires some realistic evaluation of the opponent's ability.