I believe this is the most likely reason. Remember the photo of the McLaren screen that was tweeted a couple of weeks ago? It really looked to me like the display itself was 3D because of things like the alignment of elements and the fact that some appeared to be OUTSIDE the screen. That would only make sense if said elements appeared to be above the screen and then parallax could cause that effect. Plus MS is promising revolutionary. touchless interaction really isn't that useful if you're still operating in a 2D landscape. On the other hand, if you're able to project UI elements ABOVE the screen then suddenly being able to sense above the screen and interact with said UI elements makes a LOT of sense. Anyways, I say all that because you know what would go really well with a 3D UI and 3D screen? A 3D camera (ala the Fire Phone). Dual 20 MP sensors would still mean that the phone has to process 40 megapixels, a 40 MP camera would require it to process 80 MP of data.