John Mark Walker has written a good article in which he tries to explain the economic reasons for the existence of the open source model. As opposed to a more 'idealistic' view. Also read Nicholas Carr's response and that of Matt Assay.
"I have shown how the internet has driven software prices into the dirt, created an environment conducive to open source collaboration, and provided the infrastructure for that collaboration to actually take place. I have also shown how cheap commodity software markets are necessary for open source development and how open source is not viable in less mature software markets without the necessary economy of scale. When viewing open source development from this perspective, some things become clear that perhaps were not before."