I also felt a similar way in terms of wanting to keep my day job skills in alignment with my side projects. I think its actually the opposite. Picking up python and Django (or RoR) will make me much more valuable for my own work and otherwise. Microsoft certainly has its place for desktop applications but that's not my focus. Heck, even for that, I am seeing more applications in Adobe Air.
Also, it's just fun to learn these new stacks. I don't need to be an expert, just enough to get the job done and know when to ask for help. Another motivator for me is the ability to hire developers remotely to take on development. I would imagine this is going to be easier to do with an open-source stack versus Microsoft.
Don't get me wrong. Microsoft in the last 3-4 years is doing great work. If you happen to stay out of the Silverlight, WPF, SharePoint side of things it's not been all bad. ASP.NET MVC is now mature and there is enough momentum with nuget and open-source .NET projects that you can replace components if need be.
I'm fortunate to be on both sides of the fence. I don't intend to never use Microsoft tech for my side work but there is a lot to gain from seeing what's out there that isn't Microsoft.