One potential function of a karma system I have not seen mentioned is that it can act as a release valve that gives a person a way to express his displeasure, fair or not. Without that release valve the person might instead write a flame, triggering a flame war. This is in addition to the fact that actual flames are themselves likely to be heavily downvoted. Some unfairly assigned negative karma may be a fair price to pay for relative freedom from flamewars and the tremendous waste of time they represent.