This is wider than HN-related only. Some non-tech bloggers do this as well (disallow comments). My understanding is that they try to move discussion from private space - which is their own website - to public space, which are all the social networks you can imagine.

The difference is that only people who already visit your site can see the private discussion, but when it starts on HN, Twitter, Facebook, Google+, etc, it can catch attention of additional people. These bloggers are willing to trade clarity and ability to enhance their content for chance of additional exposure.

I have no idea how well it works for them and I would really love to see some case studies (especially from non-tech bloggers, who targets wide audience with their lifestyle blogs, etc).

I believe people are much, much less likely to comment if they can't do it on the site, but they need to post it to some social network instead to say what they have to say. Is that additional share on Twitter or Facebook worth all those comments which were not written only because there is no comment form on the website?