I've been watching some high level player's replays on SC2, and I noticed that when one opponent knows he'll lose, he just leaves the game.
Is that considered a victory for the player who did not leave? Is it the same kind of victory as completely destroying the enemy (without him leaving), or is there some kind of difference (stats-wise)?
Yes.
You should receive full points. No part of the point calculation or the change to your rating is reflected by anything that happens in game.
On an unrelated note it is considered proper etiquette to say "gg" before leaving in such a manner.
Now I'm sure someone will mention pro-matches in reference to this question. When participating in a tournament with Bo5 style matches (Best of 5 games, or First to 3 wins) experienced tournament players will actually delay a final push when they believe they have 100% chance of victory. The purpose is to cause the losing player to exert additionally mental stress in trying to win an unwinnable game and thus have them "tired" for the next game in the series.
While this behavior is not considered bad manners and is often times a signifier that you are playing against a tournament pro, it leads many professional gamers to bow out earlier than you might normally expect. Because they know they still have to play potentially 4 more games, top players will often quit if their opponent has gained a significant advantage at any point in the game.
The corollary to this behavior is that in a game N situation (or game N-1 if you're losing floor(N/2) to floor(N/2)-1), losing players will often stay longer as defeat implies elimination.
