In the talk I used a simple use case: I have a website with ten thousand XML documents, contributed over several years. Find me all the documents that use, directly or indirectly, a particular stylesheet.
Today, you can refer to a stylesheet with a processing instruction, a META element (e.g. in XHTML), an attribute on a STYLE element, even an Apache server-side include (<!--#include ....-->), in a special comment.
This is crazy, folks!