I can't help but think that between this and the Android Visual Studio integration, we might see Android app compatibility on Windows Phone, much like the Blackberry app play a couple years ago.
It would certainly put the jumper cables on Windows Phone.
Adding a compatibility layer for a large competitor means:
A) users will get the cool OS updates as soon as possible, but after the guys running the 'real' thing get them.
Guess what choice users will make?
B) developers who develop for your platform have to support the market leader, too; developers who develop for the market leader get your users for free.
Guess what choice developers will make?
This tactic killed native OS/2 apps and, subsequently, OS/2. The Blackberry plot didn't end that well for Blackberry, either.
This is a good point. Few developers would care about Windows Phone if they can just build for Android and do a lazy port to WP (if they port at all). But what if Microsoft instead wants to make Windows apps work on Android? Microsoft is championing their "universal apps" now, where you build one Windows app and it runs on Windows 8/10, Windows Phone and soon Xbox One. Adding Android to that list would be big.
It seems to me at least as likely to have the "Win-OS/2 effect" of putting a bunch of apps (most of which feel really out of place and none of which feel great) on the platform while causing even fewer developers to target it natively because, hey, why not just invest all the extra effort a native version would take into the Android version since it "runs" on both platforms?
Edit: Which isn't necessarily to say that wouldn't be Microsoft's best option for Windows Phone right now. Especially since games and entertainment apps are such a big part of the app market and their "nativeness" tends to be less important.
That's what I think too. I see few other options for Microsoft to rescue WP. They should add Android app compatibility to Windows Phone and invite Android App developers to the Windows Store. And then hope for the hacker community to unofficially add the Play Store and/or unofficial ports of Android apps.
Maybe WP can regain some interest that way. Being close to Cyanogen makes a lot of sense in that scenario.
Having Android app compatibility isn't saving Blackberry and Microsoft would gain nothing from people installing Google's app store where Google control ads and apps.
It would certainly put the jumper cables on Windows Phone.