I wasn't able to capture the entire conversation in my head by reading the archives but I just wanted to chime in on how we (MELPA) see the README vs. the commentary section. As far as I know, the commentary is what we use to craft the `describe-package` information. Our impression was that for single-file packages, the commentary is normally enough to describe the purpose of the package and normal use-case.
For larger packages, lets say "magit" for example, the commentary should be a short description of the package and maybe a link to more documentation, possibly a link to the README.
I can see that in some cases this is a bit of duplicate information, but in most cases I think the commentary is a bit lighter on details.
Hope this helps give some outsider input. I would agree that "as MELPA" what we say or feel shouldn't impact the greater Emacs community. I contribute this information as a user who has thought about packages a lot.