I think people can only agree to disagree when they can see past their differences. If it's something like which are better, tomatoes or potatoes, then it is easy. If it is something deeper to one's center like PC or Mac, or some programming language then it's harder, and conflict can end up huge if it's over life and death or reputation building. In mostly anonymous and almost random meetings, such as HN, agreeing to disagree is hard, and more often than not, conflict or alliance is expressed through moderation rather than enmity or camaraderie.

At the end of the day, I tend to view argument as vulgar, but put up with it in striving for truth.