I like ESR's opinion on reciprocal licensing like the GPL and that is the market punishes those companies that take the source closed. In-house forks are probably hacks that contain specific business logic or minor enhancements. ESR best explains it in this video:
http://www.youtube.com/watch?v=gEPg2M1qbEs
I've seen libraries in-house that are open source but have been abandoned years ago by their original community. Those libraries are pretty much lost to the general public but live a happy live as closed source libraries which are maintained and fixed.
If those libraries were abandoned by the community then the significance of taking the library closed by some company that needs to maintain its legacy codebase is probably minimal or irrelevant.
