Instead of serving stateless PHP pages, you get a fully stateful AJAX application. Roundcube already uses AJAX, but crucially, not for making the IMAP connection. The proposal was about having a persistent IMAP connection, without having PHP as an intermediary. In that case you don't need to open up a new IMAP connection for every user action.
You only use one proxy, which can be written in any language. In fact, it's only there in case the IMAP server is not accessible from the internet.
The PHP side of Roundcube does a little bit more than just maintain an IMAP connection. You're basically talking about a total rewrite of Roundcube just so it can be more like another project. Use the other project :)
Cor
List info: http://lists.roundcube.net/dev/