It’s all fine and dandy to say that downvotes shouldn’t be used for disagreements (which, on the other hand, pg himself has said is fine to do), but no voting based system can prevent that. Voting by the masses by itself is a flawed system that depends on the attitude, mindset and knowledge of the voter.
There are many users on HN who downvote to express disagreement or dislike for certain comments. I too downvote some comments that I disagree with to prevent them from surfacing up because I do not value those comments. If upvoting is meant to help “useful content” (as perceived by the reader) surface to the top, then downvoting is meant for the opposite purpose. Whenever I see a downvoted comment whose content I believe I agree with or that others may have aggressively downvoted to express disagreement, I upvote them.
Nobody, in my recollection, has ever said that upvotes shouldn’t be used to express agreement. So using upvotes to express agreement while saying that downvotes shouldn’t be used for disagreement makes no sense. We don’t live in utopia. Not every comment is worthy enough to stay visible in a black font with text replies to express disagreement and go on with lengthy arguments. We are not objective beings who don’t care about that number on the top right that has no tangible value.
Looking only at the downvoted or flagged comments in this thread will show what the broader community wants or prefers.
Edit: After having said all this and admitting that I do downvote comments that I disagree with, I also believe that downvoting for this purpose is probably not a good idea for one’s mental health and emotional quotient. Most of the time, I browse HN comments without logging in (so no voting at all), and that’s a way to just observe and experience HN.