Hi Arthur,
This is a very interesting feature! Not only would it be useful for the scenario described by you and others in this thread, but I also think it would be useful for temporarily showing definitions from another place in the same file that you are editing. Sure, there are tooltips, overlays and whatnot that can do the same thing ("intellisense") but sometimes actually seeing the full definition right where you are might be useful. No need to jump back and forth, with a key you could show the definition of a function where you are, and with another key it would be hidden. You would be able to copy pieces of text from it and whatever. That being said, I have never ever seen a feature like this before, and that could be an indication that it is not actually useful, or no-one has thought of a crazy idea before the Adobe people did it...
/Mathias