RE point 6:
> It removes comments from where they should be, on the destination site.
Each place-of-commenting is its own community. You comment on HN because you want to talk to other HN folk. Generally, each community online has a particular focus. On HN you get a startups-and-tech focus, which is very different from what you get on reddit, or the Guardian, or NYT, etc.
Finally, the quality of each community is different. Contrast HN to reddit to reddit-when-it-started to techcrunch. Part of that quality comes from the software, some from the community, part from the moderatorship.
A blog is it's own community. HN is it's own community. There should be comments on both.