Personally I like the XP because I can use it to gauge whether my comments are worthwhile, or absolute rubbish. Some might argue that in the end what other people think about someone's comments don't really matter but as someone who's trying to improve their Perl abilities it's a big help.
However, more useful are the comments that go along with the voting - I did consider how much time we'd end up commenting on our voting choices if it was made mandatory (which I hope it never is). Personally if I think a node has something worthwhile but someone else has pointed it out, i'll just ++ it - if no-one's commented I might just jot down a couple of words. If I --'d someone then I (hopefully) would apply the same rules.