I have designed, and parsed, quite a large number of file formats over my 30-year career, everything from line-oriented formats like troff, to Scribe, SGML, MML, etc. I designed Adobe's PPD and EPS file formats 18 years ago or so. iMovie's project format. iPhoto's albumlist (which in fact is in XML format). Dozens of little things in between.
Many of the file formats I've designed have been in use for over a decade. Most have been through multiple revision levels and are backward- and forward-compatible (you can read an iMovie 4 project into iMovie 1, though it obviously won't understand and preserve all of what's in there).
The whole reason I've established this site is to call attention to the Emperor's New File Format and spark a conversation about information design. XML is not a good file format, yet it is widely used. Let's come up with something better.