I agree that it's fun once in a while to have your opponent make a blunder especially if you are in a tough tournament and it gives you several hours to rest and prepare before your next opponent (this hasn't happened in a while for my son, but we can always hope).  But as we watch my son's games online it's become a bit of a disappointment when his opponent blunders in a hard fought game.  Nothing is learned, sense of accomplishment is hollow, and rating points are undeserved.  It may be fun, ironic and sometimes just plain stupid what people will do, but ultimately you are the one that may be robbed and no stronger in your chess because your opponent had a momentary lapse.