But you never give any reasoning for your own disproportionate weights beyond what you personally feel is fair or seems to work. Granted, HN gives no little rationale as well, but that makes it at least as valid, having survived longer. It may be that the ideal weight for downvotes is even lower than 1/5th, or it may be that merely having downvotes at all hurts a site more than not having them.
The message an upvote sends is "I like this and I want to see more of it". Conversely, you seem to think a downvote should say "I hate this and want to see less of it", but when the site adds them directly to upvotes, the downvote message becomes "I hate this and I think everyone who voted it up is an idiot." A site using this mechanism sends the message "We are willing to allow controversial material as long as an enthusiastic subset of users votes it up." I would expect this to encourage clique voting over time.
By contrast, the message the flag system sends is "I don't think this belongs on the site; what do you think?" It defers the decision of what belongs on the site to editors. This is more work than letting the market decide, and it probably doesn't scale as well. However, it also ensures that the market doesn't take your site in directions that you don't want to go.