FWIW, I think it's great that AOSP can be used this way but GMS requires certification.
Customers on Amazon and Nokia X devices are very different from the typical GMS customer and it's reasonable to treat them differently.
The Amazon device is a great example of how we can get subsidized hardware in the hands of everyman outside the US mobile phone system. I have hopes that these types of devices can help us bridge the digital divide. That AOSP allows for its use in this way is key.
Regarding Microsoft's use. I have opined previously that it's time for Microsoft to abandon windows in favor of a customized Linux or Unix distribution. Imagine the things they could do by freeing up the manpower associated with upkeep and development. The same concept holds true here. What fundamental advantage can the Windows Phone operating system offer the customer or the software developer? Obviously it can't be used on entry level devices. Why not, instead, focus on a Windows Mobile Services layer that brings this functionality to a range of phones with less effort? Again, it's not fragmentation when it's marketed as a different platform and the user doesn't expect an upgrade path.