That all makes good sense for an app built by a single developer and a specific demographic. He's cutting 10% of his potential market and probably saving 80% of front-end dev time. Seems like a good tradeoff.
Quite often it's worthwhile maintaining a bare-bones version of your site. For more than just compatability - there is also accessibility, printing, slow connections, etc, etc. Surely it's a better user experience to just downgrade them to that?