With the greatest respect, you're clearly looking to create a problem by concocting a special test to do so. I never, ever have a fully grey screen on my computer. If I did I'm sure I'd see problems on any screen.
The only problems that we should be worrying about are those that are obvious during everyday use. Otherwise you're just giving yourself an unnecessarily hard time.
This isn't about creating a problem. Theres's nothing special about it. It's about isolating issues and illuminating them properly. You may never encounter a totally grey screen at one time, but throughout the day, your entire screen eventually renders grey, little by little.
This test isn't like using an all black screen in a black room, with the iPad at an oblique angle. That's looking for trouble that doesn't exist in real usage.
Grey best represents the elements of the iOS UI that people most often complain about being discolored in every day use. The keyboard is grey. Safari's address bar is grey. etc. When people complain about black and white photos having an odd color, it's the discolored greys they're complaining about.
Grey also easily shows uneven illumination, which many have said they detect, but are unable to pinpoint.
People have been posting all white screens as tests since day one of the discussion of the screen issues...no one has claimed that that's unrealistic. But the fact is, there are grey elements on many areas of the screen at a given time, and if there's a discoloration or illumination issue, grey simpy shows it more easily.
Grey is the visual standard by which color is judged to be accurate. That's why photographers and videograhers use a grey card for white balance instead of a white card. That's why color correcting software instructs you to sample a neutral grey area to auto-adjust the entire image.
And as has already been shown in a recent post, using a grey test can be positive...the fellow posted that this test showed he had a problem free screen. So it's not just about finding flaws.
Lastly, this test isn't for those who think people should just ignore screen issues and enjoy their iPad. And yet oddly, they're the ones who discuss the test the most vehemently.