The idea would be to lower the barrier for current Android users to trying a Windows phone for their next phone, because they could keep their Android apps.
You need to lock developers to your platform whichever way you can if you want to survive.
I didn't recommend it since it isn't free, but it's well worth the money if doing cross-platform apps (they also give students a reasonable discount). If initial cost is a concern, Scala would be my alternative choice.
About a year ago, Xamarin did a direct port of the entire Android operating system to C# as a research project to improve their automated tooling. It's called [XobotOS](https://github.com/xamarin/XobotOS). It's open source, unmaintained, and entirely unsupported.
I was hoping to mess around with it and make some sort of compatibility layer for dalvik based apps to run on it and everything else to use C# that I or anyone else might do (I mod AOSP and such in my spare time).
Too bad it Xamarin has not done anything with it since the announcement, as dalvik is the bane of performance on Android.