InformationWeek has an interesting interview with Linus Torvalds on the why of Open Source Software.
Torvalds: I think the real strength of Linux is not in any particular area, but in the flexibility. For example, you mention virtualization, and in some ways that’s a really excellent example, because it’s not only an example of something where Linux is a fairly strong player, but more tellingly, it’s an example where there are actually many different approaches, and there is no one-size-fits-all “One True Virtualization” model.
I completely agree, though I would add one other reason – control. Open Source Software delivers more flexibility and control. It empowers developers, IT organizations, businesses and end-users. No more waiting with your fingers-crossed for the next release from a proprietary vendor – praying they will include a much-needed feature. With OSS, you have the flexibility and you control your own destiny.