In the document, it says "Based on the limited data, we are undecided about whether this should be a requirement." which is hard to understand. Even image maps need data sources, and geospatial maps are not different. Just because the big mapping providers don't support mixing data sources doesn't mean it's not a requirement for the Web. By analogy, just imagine if you could only use YouTube for videos in Chrome, etc.
@prushforth "Undecided" is the default conclusion from the template (found towards the end of index.html with id "capability-template"). I'm currently working on fleshing out the use cases, required capabilities, and examples for the Client APIs section, and will be updating the conclusions accordingly. And as you say, this one in particular will be a necessity :-)