Seconded.
A web service API utilising core_courses_get_categories that I had built stopped working because a field was "optional" on a different installation of Moodle. There simply isn't any mention of "how to get optional fields via the web service API" from my Googling. Thanks for your post.
Add to this that there are probably 4 different ways to pass API requests and no clear documentation, and it seems Moodle is even more inconsistent than PHP vis-a-vis function names and methods.