I used to be opposed to open source, but over the last year, I've grown to prefer it for a number of reasons:

• People actually get to play with your code.
• If everything is open source, nobody has to be faced with the compromise between quality and cost.
• It's fun to show off your code and get feedback, potentially inspiring others.
• It adds a competitive element: Others can copy your code, so if you plan to use it to make money, you are forced to continuously improve upon your own code.
• Collaboration.
• The goal is the product, rather than making money. Let's face it, the most impressive and inspiring things aren't always those which provide the most business value.
• It adds an element of decentralization.