See the settings in the blog for Show Page Left Content and Show Page Right Content, it will show the content in the viewpost. It does not currently show in viewcategories or viewarchives but I will make it do that for the next release.
There is work planned to make a new comment system and new category system in the core that can be re-used across features and I will be replacing the internal comment system in the blog with the new on when it is ready and replacing the categories with the new system. Then it will be possible to use the comments and categories in custom features without having to re-implement it for each feature.
Note that the external comment systems like IntenseDebate are based on the url not on a post id so a post really needs a unique url.
Hope it helps,