Earlier quoted context omitted.
> if they were due to perceived ignorance or stupidity, then I accept them You were just asking an earnest question. What's actually stupid is the downvote feature itself. Upvotes can help promote good content and flags can call attention to inappropriate or problematic content. Likewise, comments can be used to express disagreement. So, what's the purpose of downvotes? Are we in preschool?
If users should be trusted to push comments up, then why not down also? A down vote doesn't have to mean "this comment should not be on HN", it can be "I disagree with this comment" or "I think this comment should be lower down".
You actually get two downvotes: if you think a comment should be lower down you can (but shouldn't) upvote all the other comments and downvote the one you think should be lower.
Downvotes are an optimization.