Technically it's relatively trivial, but it makes the App less idiot-proof. We'd need to change the color of the panels in design mode so they'd show, but then it's not WYSIWYG. We do this in other places -- ControlViews that are copies, for instance, or hide-in-play mode -- which does help to make the case.
Anyway, thanks for the suggestion, it's on our list. Best!
Dan