I disagree that a vote down fulfills this function. A vote down does not say "i disagree", it says "I want to see less of some feature of this comment/article in the Less Wrong stream".

Sometimes that's because I disagree strongly enough to consider it foolishness and not worth discussion. But most of the time, a vote down is for other reasons. I do find that I am much more likely to vote down comments that I disagree with, and I suspect this is true for most/all Less Wrongers. But that's because I am more likely to be looking harder for problems in posts I disagree with due to all the various biases in my thinking. Disagreement alone is insufficient reason for a vote down from me, and I hope that is true for almost everyone here.