In my experience, looking at it from the other side is even worse. In fact, it's so bad that as soon as they hear Microsoft their brain shuts down. They get this emotional reaction that makes any rational discussion difficult if not impossible.
If it was merely zealotry, then I'd be okay with it. As you point out, there are zealots on both sides of any divide. No, the problem I'm talking about is ignorance. On the Unix side, people have at least heard of other frameworks. If you're a Django dev, you've probably at least heard of Rails, Plone, etc. If you're on the Microsoft/ASP.Net side of things, however, it's routine to encounter developers who haven't even heard of Django or Rails.
I wouldn't say zealotry is any better. I've had experiences where as soon as anything MS is brought up, or they find out you use MS technologies, you get looked at like you're crazy / inferior. Zealotry is a good word for it, because I'd get a similar feeling to when somebody finds out you pray to a different God, or even worse no God at all. In my opinion ignorance is easier to fix than zealotry.
Zealotry will do less damage to your career than epistemic closure.
If you had to choose one of two faults: Nothing But Microsoft OR Anything But Microsoft -- you're going to be better off as an ABMer since (theoretically) your ecosystem of options is MUCH larger. Rejecting Microsoft still leaves you with several other marketable options. Rejecting everything that isn't Microsoft leaves you completely at the mercy of a single platform that you have no control over (and if you did have control over it, it still wouldn't be ideal)
You're making the assumption that MS is the only target, it's not. The reason it's so destructive is because you eliminate certain options based on emotion rather than reason.
So on one hand you're dealing with a fanatic that only sees black and white, and the other you're dealing with apathy. Not exactly good options.
I used to find that some years ago, but not so much any more. Microsoft used to have a reputation for making a lot of quite shoddy software (VB/VBA/etc.), but most developers, even the most hardened OSS neckbeard-wearing types will admit nowadays that .NET isn't a bad framework and C# isn't a bad language.
