do you know how people earn through open source ventures? When free and open source projects help many in great ways, still I think that the people who get involved in it full time (not volunteers) should finance them selves to keep the project going.
I am asking this question primarily as I didn't have a good explanation to one of my friends who asked me the same thing; he was going to build a GIS system and was planning to move it forward as an open source venture. At the same time he was looking to generate revenue from the project to fund the development costs. According to him his primary intention was to make money out of the project in the long run, and I was clueless about the viability of such an effort.
My understanding is that opensource projects primarily make money through charging for support for enterprise clients or writing custom code. If you have something like wordpress, with a large userbase, then you could also offer value added things like premium hosting.
It's doable in some circumstances, but they seem to be the exceptions...
What you sell with regard to open source is expertise. Let's say that you give for free a tool that allows for the automatic verification of complex systems. How to use it to actually verify the correctness of a system is tricky since you have to model your system into the input of the tool. Once everybody is using your open source tool, that have to learn how to model systems, so they need to buy consultancy to learn, then to assist them. Trust me, it can be very profitable.
From the marketing point of view, it is even better since once people use your tool, it is best for them to pay the firm that design the tool for assistance than any other.