If all windows phones ran all android apps, why would anyone write apps for Windows phones? That's especially true for the long tail. A popular a Windows phone OS with Android support may get a Twitter app and a Facebook app, but your bank's app or that public transport one? Forget it.

We have seen that with OS/2 and its Windows compatibility layer, with Mac OS when Apple advocated Java, and in some sense with Linux and Wine (a great effort, but it does give people an excuse for not porting their stuff to Linux.)

I think they would be better of writing a launcher for Android that launches Windows apps. If that gets popular, they could start leveraging that by releasing new features on their hardware first and on the Android layer later. Good idea? I wouldn't know, but the reverse certainly isn't.

A native compatibility layer does (almost) everything the 'real' system does, (almost) equally well. If it doesn't, people will be disappointed with it, and by extension, with your system ("yes, Windows Phone can run Android apps, but some will crash, and that game does only 20 frames per second")

If/when HTML can provide the same experience as native code on a mobile phone, many of what now are apps will move to web-only.