I think my use case was Sharepoint and static site generators - Sharepoint can serve a tree of .aspx files as raw HTML, but maddeningly not .html files. The site generator we used spit out a fairly complicated site where the internal links point at directories counting on the fact that it is served correctly by the directoryindex handler. The site generator spits out .aspx but http.server can't serve them. 

A directory full of .xhtml files would have similar problems, which certainly still exist in the wild. 

The example in the test was named index.test because it's creating files on the disk in the test, shared by other parts of the test, and I wanted to be clear about what it was. It was not intended to be an example use case.