Yes, we expect users of ANY platform (including windows) to benefit from our services. So yes, a subscription is useful.

But you certainly won't need it. You can write your game on Windows and publish it on your own without ever contacting our services, or you could use our free services on windows and also never pay a dime.

You could also go ahead and do all the hard work to port your game to another platform like Windows 8 (see license, as long as you don't publish any source code and don't use our platform code in your code base). Honestly it is not that hard, especially right now as we only got a few thousand lines of code. But the question remains: Why would you do that?

Currently in the beta ALL services are free, you do not need a subscription to build for Windows Phone or Windows 8 and even if the beta is over it will only cost you something like 5$/month.