I was reading the book Open Source Development with CVS (here) and found a excellent rationale for open source software on page 23 :

“Traditional capitalism is based on the idea of limited supply; however, information has become a commodity in itself and is never in short supply. In fact, the ubiquity of computers and the Internet has made it possible to replicate any information effortlessly and without bounds. Even so, we still treat software as if it were a tangible object in short supply. If you copy software from somebody, you’re legally stealing it. The software industry has attempted to extend this metaphor into the information economy, artificially re-creating the economics of limited supply by creating software licenses.
There’s nothing wrong with making a living as a programmer or as a software company employee or executive. The authors of this book get part of their incomes as programmers. However, it’s nonsensical to use this profit-model. Imagine a science-fiction device that allows any sort of food or physical object to be infinitely duplicated. If somebody then tried to sell you a tire for your car, why in the world would you buy it? You could just throw your friend’s tire into the duplicator! However, you might want to pay somebody to design a new tire for you or perhaps to install the tire on your car. Or to help you when some other part of your car breaks, you might want to buy a warranty for future support. Or maybe just hire a personal mechanic.”

I tend to agree with what he is saying.