I think the rule is in place because there is no telling what anybody is eventually going to put on the shelf. Even though, in your application, you are reasonably sure that nobody is going to start stacking a bunch of crap there, I think it's safe to assume that most people would. Give people a shelf, and they'll use it. As I'm sure we all know, people have little regard for electrical equipment, and often like to cover up that "ugly THING" on the wall.
