For me it depends a bit on the circumstances. I actually built my dice game website because of getting annoyed back in 2006 by people quitting early in games such as carom3d, FIFA 05, even Tribes 2 (FPS) etc. Cheating, quitting. Meh.
I think there is a time when it has a place. There are games where it can become almost impossible to win the game so I think it's reasonable to ask the opponent's position permission to quit. I have done it in draughts or what's it called, checkers? I chatted and told my opponent there is no way back in to the game, would he mind if I resigned? He said go ahead. I congratulated him on outclassing me, not much of a challenge I am afraid, and thanked him for the game. No problem. In chess, resigning is fine too.
Obviously sometimes things happen in real life and we have to abandon the game, also totally acceptable.
But any other circumstance, when there's even a one-in-a-million chance of winning, players should absolutely continue, or if it's a game where others are doing cool things and setting-up cool combos, let them try them out, finish the game, or if it's just a couple of minutes, play it out.
In real board games if it's a two-player game I'll often tell my opponent how I see the end of the game unfolding if it's an obvious loss and ask them what they think - if we agree, we can end the game, give them the win, then start another game. Anything else, you committed to spend the time and asked somebody else to do the same, so stick to that agreement.