I strongly believe that they are a part of the content, just not in the traditional sense. Many times I find more substance in the comments than in the main content. Sure it's great to have more control over which commenting system to choose, but would adding a native commenting diminish that control? Certainly not right? (If you check out ghost's forums, you will find many people asking for a native commenting system)

Yes none of those static site generators support commenting, making them not very suitable for blogging(they dont even support searches). That is why I am not a proponent of static generators as a blogging platform.