Divide and conquer.
By making any success with its Android phones, Microsoft can achieve two goals:
- further fragment the Android platform making it more difficult for users to buy devices and for developers to support the various devices and
- benefit from Android's strong market position instead of only trying to create market for Windows phones in places where it doesn't fit
They've identified the greatest flaw in Android ecosystem and they're exploiting it in an attempt to make their own offering more attractive in the long run.