Given the recent discussion, I'll ask the inverse and naïve-sounding question:
Why would anyone today sink money into a Microsoft solution for developing a web-based service or application?
I mean, if you're already up to your eyeballs in Microsoft contracts, I could see momentum being on their side. But, assume you're just starting today fresh from a good idea. What would Microsoft give you today?
The only thing I can really think of is support. As in, someone to call and bug for answers or blame for problems. Because—and maybe this is just a demonstration of my own ignorance—I can't really think of an area where Microsoft's offerings really blow any Open Source platforms out of the water, especially not once you've considered cost and the loss of flexibility.