For what its's worth, I think that open-source is no panacea, and in fact is one of the biggest black-holes sucking away human talent needlessly these days. How many man-hours have been spent building a clone of the 30 year-old Unix operating system? There are many better areas for us to be applying talent. And I don't mean to diminish the professionalism of Microsoft developers. The product teams here are some of the most well-tuned machines I have ever seen, but "best" is not the same as "perfect" or even "as good as possible".
(BTW --- What's the difference between "perfect" and "as good as possible"? Nevermind.)
And why should one develop an open source Unix clone? For one thing, because one can! The ideas are well known and successful, which makes for the best patterns and lowest risk. That's what's known and now accepted as pattern-oriented software development, and so most software development in general should be like this. See Eric Raymond's book, The Art of Unix Programming.
There are many better areas for us to be applying talent.
But going all the way back to Stallman's instantiation of GNU, clearly (in hindsight!), there is a need for a platform for innovation. The platform should be well known and successful, but also unencumbered by proprietariness or the unspoken requirement to abide the vendor's cash cows.
Why try to innovate on a platform vendor whose not-so-implicit intention is ultimately to own any and every idea that succeeds? Enough said there, even so, "Why Unix?" should be *obvious* to a software developer for many reasons. None of which are, or need to be, "Because it rocks!"