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.
For this application, where you might have a lot of authors and apps working with the rule data, I think schema-based validation at some level is going to be a must if you don't want to end in sorrow.