I came across a couple a draft paper on dual-licensed software by Mikko Valimaki. It examines Sleepycat (Berkeley DB), MySQL and TrollTech (the Qt people). One of the observations that Mikko makes is that Berkeley DB, MySQL and Qt are effectively proprietary software packages. That is, each of these products are developed, maintained and enhanced by the company that owns the copyright; they make money by offering commercial licensing for commercial use. Each of these three companies is making a significant portion of their revenue from licensing the software that they have created. Accepting contributions from the community is nice in theory, but impractical for a number of legal, cultural and engineering reasons.
All of these companies started in the 1990's. Part of their business model might just be playing the role of a "good network citizen" as Tim O'Reilly would put it. But maybe there's something else going on.
It's difficult to start a proprietary software business today; the costs are very expensive (you need a large staff, and very good marketing), and the threat of monopolized out of business by Microsoft is sometimes very real. The alternative is to embrace the open source community and let us adopt your software package. We'll let you know what's good, and let you know what needs fixing (and occasionally provide a fix). We'll be an extension of your marketing department (you can't swing a cat without hitting a mysql user at a tech conference these days). And we'll even help with the documentation (identifying what's weak even if we don't write your docs for you).
What results is a partnership. For people who are willing to use the open source licensed version, all that's needed is a little sweat equity. For people who don't want to pay with sweat equity or demand more flexibility/support/features, there's always the option for a commercial license.
