It's just a toolset. If you're smart, you can fit more than one language or platform up in your noggin and be able to move back and forth. That said, it's a tremendously powerful framework. If I may say so, as someone who goes back and forth between C++ and .NET, I find C++ to be akin to a wrench, hammer, and screwdriver. .NET feels like a compression wrench, a jackhammer, and a power screwdriver. Sometimes, it's a little too much, but generally for most things, I get more done quicker with fewer lines of code, enabling me to spend a lot more time polishing the edges and handing off a more polished product to my customers than I can with C++ or even Java (in some circumstances).
The reason I do it, though, is because far more CEOs than some random, obviously big-headed CEO of some startup pay me to write code in that language. And I don't just do "Windows Mobile" for god's sake. I write a lot of web services, and the majority of customers don't care what's running on the server...they couldn't care if you wrote that service in C#, C++, Java...using SOAP, REST, whatever. They just want data when they click a button, and they want it to be up to date. There might be a few nerds who have the opinion that they might get their data a tad faster if they used one or the other, but frankly, over time the more maintainable project will always be the one that works the best...because developers will want to work on it, refactor it, keep it up to date. If you write it in some obscure fad language that has shaky support, guess what developers will do to it once you get another job? They will throw your work away and rewrite it from scratch. And I guarantee that, with the abundance of .NET developers out there, they're going to rewrite it for .NET anyway. And they're going to curse your existence the whole time. How does one such as I know? Because I've been on both sides of that mess. I've written code in goofball languages when I was younger, and I've been tasked to replace projects written in goofball, non-widely supported languages plenty of times over the past several years.
And then there's the last thing. I put my support behind a framework or company that provides powerful tools to developers. It's sort of my activist side coming through. If a company could care less about providing powerful, open, extensible tools and frameworks for developers, that's a sign that a company simply doesn't care about the quality of third party products for their platform...and a company that most likely would use their internal tools to trump any project that I'd put substantial effort into that managed to become popular with users. For example, if I were to make a killer cloud service app for iPhone, and Apple saw a substantial potential revenue stream in it, they could easily use their own internal use tools and APIs and put me totally out of business...which is a big reason I don't do iPhone development anymore. And it's also a reason I am highly tentative about doing Android development either, because for all the tools Google puts out, I know they've got better ones behind the scenes and I've seen them kill startups plenty of times by releasing slightly better (or even inferior) services for "free". And when I say all that, I'm also saying that I want platforms that aren't just open and follow standards well, but I want a platform and toolset that is well integrated. I want to drop a breakpoint in my presentation layer code, drop one in my service code, and drop one in my business logic layer code, and I want to debug through each layer to find a problem. I can do that with .NET because frankly, Microsoft put forth the effort to make that really quite simple to do. I don't like wasting time unhooking my business logic from my web service and hooking it up to my presentation layer just to run a debug, and if I'm not getting a good stack trace, then the toolset I've been given is crap.
While I'm certainly no Microsoft fan, I certainly want to support their developer tools as a way of saying to all those other evil overlord software corporations out there "Hey! They're doing this right...why not follow that example! You don't have to be just like them about everything, but on this thing, they've got it right and they understand us!".