> We were looking at joining and combining diverse datasets as a key use case for XQuery.

This technical requirement is generally fine.

>   It's possible that we missed something,

Do preferences matter if combinations of information sources are performed with a key word
like “JOIN” (or not)?

> but it would be extremely helpful to have a concrete use case to consider, with real data.

Would you like to check the influence of numbers according to joinable items
once more on data processing efforts?