In my opinion, XP is a measure only of the popularity of a post, not of its value, correctness, relevance, or anything else. I have often seen intelligent, pertinent comments downvoted because they displease people - an example might be a post that discusses shortcomings of Perl. Similarly, many enthusiastic but essentially content-free comments are upvoted despite having little to say besides, "Perl Rocks!" (which it does - but that's beside the point).

Ideally, of course, we'd all vote objectively on the content of the comments, judging only on the accuracy and usefulness of the posts. And there'd be an end to world hunger, war, and sickness, too. Unfortunately, we're moved by less noble motives and we often yield to the temptation to downvote unpleasant facts...

XP is still the best system I have seen for encouraging participation, checking facts, and interacting in a friendly, cooperative way with each other. But it's not really a good measure of anything other than popularity.