This is a good question! We do it, it works, and it's definitely an advantage of XML over alternatives. I just personally haven't had the time to dig in and learn it well enough to write a blog post about it. In practice I think people update the Fact Dictionary largely based on pattern matching, so that's what I focused on here.

I used xml and xpath a lot in the early 2000s when it was popular, and I never wrote or learned about schema validation. It's totally optional and I never found a need for it.

It's probably helpful for "standard data interchange between separate parties" use cases, in what I was doing I totally controlled the production and the interpretation of the xml.