The idea isn't to open the server to the world. The RFC states that it's meant to be used in a similar way to django's manage.py runserver, for development and internal testing.

Django phrase it in their tutorial as:

"We've included this with Django so you can develop things rapidly, without having to deal with configuring a production server -- such as Apache -- until you're ready for production.

Now's a good time to note: DON'T use this server in anything resembling a production environment. It's intended only for use while developing. (We're in the business of making Web frameworks, not Web servers.)"

As long as the PHP documentation includes similar caveats and guidance, an HTTP server isn't a bad thing.