In my opinion , in the discussion around google authorship there is too much emphasize about main content writer authorship, but almost no mention that the content indexed by google is made up also from comments and the have authors as well.
This is even more obvious in forums and Q&A sites. Who is the author of a page on stackexchange, the one who asked the question or the ones who supplied the answer. It is even more complex in wiki sites.
It feels like while people were rushing to see faces on the search results hoping for some SEO juice, they haven’t tried to reed the text of the spec
authorkeyword indicates that the referenced document provides further information about the author of the nearest
articleelement ancestor of the element defining the hyperlink, if there is one, or of the page as a whole, otherwise.
And article elements are not necessarily the whole content of the page (emphasize mine)
articleelement represents a self-contained composition in a document, page, application, or site and that is, in principle, independently distributable or reusable, e.g. in syndication. This could be a forum post, a magazine or newspaper article, a blog entry, a user-submitted comment, an interactive widget or gadget, or any other independent item of content.
But google right now will not let me claim authorship over my comments . The way you need to configure your profile to claim authorship is just not user friendly enough to do it for every site I comment on, just too much work.
Why would I as a site owner wish to let commenter claim authorship on comments? because if I have quality commentators people might come to my site because they follow them.