I don't doubt all you say is true. But consider someone (like me for example) with a Unixy skill-set. I know PHP, MySQL, PhpMyAdmin, Python, Django, Unix text processing tools, etc, and while the MS equivalents may well be very good, when you factor in my time learning to use them, I'm not going to be as productive.
One of the reasons I know all these tools is that getting started is simply sudo apt-get install whatever and I'm off. If MS want a higher uptake of their developer tools, they might make them as easy to get at as that.
I understand that with BizSpark I can use Microsoft's tools without paying much money. But there's a bureaucratic process to getting on the BizSpark program, which I'm guessing takes days or weeks to achieve. By comparison, sudo apt-get install takes seconds.
And that's why I'm using Unix-based tools in my startup.