Progressive Enhancement is still important for CONTENT SITES

Why? Search Engine accessibility.

It used to be that Googlebot wouldn't find content loaded asynchronously, or links that rely on Javascript. Now it's different - You can confirm that Googlebot discovers a lot of Javascript links using Webmaster Tools: https://www.google.com/webmasters/tools/home?hl=en

BUT - There's still no way to break from the "Page Paradigm" - Google needs URLs to send searchers to. They don't yet send people to specific states of a page. That's why I still use Progressive Enhancement, it forces me to ensure each piece of content has a URL that points to it.