Even if you do your best to follow the rules and code portably, you still need to *test* your code on any weird platforms you care to support - such as Windows - just to make sure that you *did* follow all the portability rules and not miss anything. And there are plenty of cases where, to make your code portable, you need to write just a few lines of OS-specific code. There's at least two of my modules - neither of which uses XS, by the way - for which I will find this very useful.
Sure, I could just run Windows in a virtual machine, but that requires an expensive Windows licence, and that I know how to run Windows securely, which I don't.
Bravo Microsoft! Bravo Adam Kennedy!