I remember this for the Windows Phone from several years ago, when Microsoft was making a big push to get a base of phone apps available. From what I saw, it was a beautiful package.
The problem they were having then is that apps just for the Windows Phone lacked a big audience, and it was difficult to port these to iOS, and, even more difficult (because of issues with Android standardization) to Android. So it was hard to justify starting out with an app aimed for the Windows Phone.
This is undoubtedly one of the reasons MS has been trying to get a unified interface for the desktop and the tablet and the phone. I have no idea whether the economics will work out. Somewhat analogous to MS in the game market, I suspect.