[Stuart]

The 'genuine reflections' point is also particularly interesting. You're right that you can have more reflective elements as part of the physical set, but the way that those physical reflective elements interact with the virtual set will often be less realistic. If you have a black glass desk top, as per ITV, and you want it to reflect a skyline as it would reflect a genuine one out a window, you can't achieve that effect using an LED wall as the physical desk can only reflect what is shown on the screen. In a VR environment, you combine the reflective technique that Stuart mentions above with a virtual skyline cyc and get a fairly real reflection.

After burrowing down some Google rabbit holes, I managed to find a picture demonstrating why the ITV News 'glass desk top' has to be CGI overlay.  There is simply insufficient uniform reflection of the green background strong enough to create a sustainable image of high quality. It would just keep fading out at the edges.