Whether it is 'worth it' is dependent upon what you want to achieve. Are you looking to get a job from this learning investment? Are you trying to broaden your knowledge by seeing how different technologies solve different problems? Are you just looking around to mess around with something new for the fun of it?
My read of your post that you are focusing on the first option, to help landing a job. In which case, as others already stated, it really depends upon if the companies using these technologies are ones that you want to work for. In general C#, ASP.NET and the Windows platform are more popular in the Enterprise crowd, not so much for startups. If you want to work for startups then focus on technologies popular with them, which tend to not use the Microsoft stack. If you want to work where-ever, as long as its good than focus on what's the most popular.
As a former hiring manager I do want to point out one thing that less experienced people sometimes gloss over; technology is always changing. So even if you are really good with the technology that your prospective employer is currently using, that doesn't automatically mean you'll be as good with whatever they are using in 6 months, a year, etc. Depending upon the place they put more or less importance on that adaptability.
The reason I think this is relevant to your post is one of my favourite personal example of this issue, which is kind of the opposite of your question. Years ago my team hired a developer whose main experience was C++ with some C#. He had never written a line of Java in his life. What was his first big feature? Design and implement a complicated drag-and-drop dashboard UI...in Java. And you know what? Not only did he do a fantastic job, we weren't that concerned he wouldn't be able to. Why? Because his background and experience had taught him enough about OO programming and software design patterns that a brand new language barely slowed him down.
So to loop back to what I think your question is: is it worth it to learn MS technologies when you don't think you want to work for companies that use them? Well, one reason to do so is to prove that you can learn new technologies and that you are not a one trick pony. Whether its from MS or not is beside the point.
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