It seems very analagous to a FLOSS-only desktop experience (for a while I think Fedora Linux was one). It was not a polished user experience, and certainly some things were lacking, but when I used it I was able to accomplish all the things I needed done (and face it, most of those can be done by the web browser anyways).

The other reason I have hope for this is most of the non-FLOSS functionality is available at least in some form in web apps, or in a F-Droid application. Google Maps, Gmail, Twitter, Facebook, etc all have pretty good (capable, functional) mobile web applications, and all you need FLOSS for is the web browser.