why does ASDF ask to please only define system/test?

It seems that any system Y associated with a name X must have its name be of the form X/Y. For example, when you build "cl-ppcre", you get this warning:

Please only define "cl-ppcre" and secondary systems with a name starting with "cl-ppcre/" (e.g. "cl-ppcre/test") in that file.

I've seen this complaint for quite a few systems already. What's the need for this, and is it really worth nagging users of all these systems that have existed in many cases for years and have worked perfectly well without following the new convention? What great functionality are we getting for this?

Thanks, -Mark