Seems like the built in server would be most useful for newbies to get an application running. Think if how much easier it is to get a basic Rails app running to start learning compared to configuring Apache and PHP together.
It isn't too hard for someone who is already familiar Apache. On OS X, mod_php is not enabled by default, requiring new users to dig in and learn about apache config files and document roots. I'm not saying it is difficult, and it is obviously important to learn, but it would be nice to be able to have a 1 page "get started with PHP" tutorial.
Having webrick out of the box was awesome when I first started using Rails. The big difference, however, is that Rails includes a web server because Rails is a framework for developing web applications, while this proposal seems to want a web server included as part of the PHP distributable. PHP isn't exclusively used for web applications, so I don't think it makes too much sense IMO.
