Dries has been commenting recently, both on his blog and elsewhere, about one of the chief advantages of using open source: All developers/users are on equal footing. If you try to learn a proprietary app or framework, you know what the main developer feels like deigning to let you know. Anything else is either a mystery or, in some cases, illegal for you to find out (if there's any encryption or copy-prevention involved). You can never be as good an expert as the author, because the author has access to the Holy Book (code) and you don't. With an open source project, everyone gets the same access to the code. The only thing stopping you from being the best expert on the planet is your own skills and time.
He's very right about why you should choose to use an open source project. But what about why you should start one, or release your own code open source? As a developer, that's a far more interesting question for me.