I agree Ghost needs and internal commenting system. Actually it is mandatory if it really aspires to become a professional tool.
As a creator you need to be in contact with your audience and “likes” and “comments” are the modern way. (even this forum have likes).
The true method to get good ranks in SEO are comments and shares. Ghost should manage trackbacks and pingbacks too.
Comments are so important that big editorials charge you just for commenting ie Condé Nast
If Ghost goal is to make it simple for publishers, it must have the tools in its system, instead of forcing them to manage one vendor per feature.
Even Facebook and LinkedIn can be used as Blogs and they have all the tools that people loves so Ghost needs to be one step forward. The membership tool is good, but it will need more to make professionals to change, even from WordPress.
I may advice the Ghost team to get the point of view of professional creators/publishers. Now with the hosted service the technical side does not matter any more.
It is not the same to upload a page with a disquss widget than making money with your content.