This game I played today, my opponent was two Colonies ahead of me. But man, my strategy was better, I had just taken the rest of the Curses thanks to my opponent's Torturers, I had finished draining the Farming Village pile with Workshop, and I had just used 3 Horns of Plenty to pick up King's Court/City/City. These Level 2 Cities were the only source of +buy, and my opponent had none. Which means I could safely drain the City pile to 2 if I wanted--he had no way to gain more than 1 thing per turn. Which also means I had him totally defeated; I could afford perhaps 2 more turns of building before a huge Horns/Kings/Cities/(and Bishop!) megaturn got me all the rest of those points. And I had planned this, I hadn't panicked when I fell behind in Colonies, or when I had accepted 3 Curses.
To play Devil's Advocate here...
An opponent who is already going Colonies who just gave you all the curses on a torturer board without buying any cities is probably newb enough to not recognize what you are building up to as an epic comeback. I know I wouldn't. I would think I've got this game in the bag. My first thought is that it was just a disconnect rather than a quit...maybe. Still your point about getting a rating boost when your opponent quits is tops on my complaint list too.
BTW
Devil's Advocate
$6 Victory - Reaction
4VP
When another player plays an attack card you may reveal this. If you do, you are unaffected by that attack, trash this card and gain a Duchy.
Edit: changed cost and VP