I don't think 2) and 3) are really valid reasons, as services like Disqus solve exactly these problems. If a blog's target audience is in fact overlapping with

I don't think 2) and 3) are really valid reasons, as services like Disqus solve exactly these problems. If a blog's target audience is in fact overlapping with the HN audience, then I'd say it kinda makes sense if the blog author wants to encourage the HN dicussion spirit. :)