It's not for you to decide what down-voting is for.

Some people don't like lies, other people don't like off-topic posts, other people don't like fallacies, and other people just don't have time to write a disagreement post ... especially if the assertions made in the down-voted comment aren't worth bothering with.

And if down-voting should have a purpose ... then it should definitely be for filtering out people not in HN's demographic.