They have open-sourced quite a lot (aspnet webstack, reactive extensions, etc). The problem is that a lot of their compiled binaries, delivered via nuget packages have been restricted to windows. This is slowly being undone ( http://blogs.msdn.com/b/dotnet/archive/2013/11/13/pcl-and-ne... ).
Another problem is that they don't run their open source projects in an open manner. Contributing requires a CLA and a signature from your boss. Issues and planning are often behind closed doors, but that seems to be slowing changing too (see KatanaProject / signalr).
If they really wanted to grow their ecosystem 10x, and give windows phone the possibility of a fighting chance, making VS Pro free would be the required.
Another problem is that they don't run their open source projects in an open manner. Contributing requires a CLA and a signature from your boss. Issues and planning are often behind closed doors, but that seems to be slowing changing too (see KatanaProject / signalr).
If they really wanted to grow their ecosystem 10x, and give windows phone the possibility of a fighting chance, making VS Pro free would be the required.