This is a difficult question, because the key distinguishing factor between proprietary and OSS is that OSS desires to be cooperative and share code, while proprietary desires to erect barriers to market-entry by withholding code.
While a proprietary enterprise may be quick to selectively adopt OSS for its own use -depending on the license - it doesn't have to share its own software inventions--and usually doesn't want to. Would such a proprietary code strategy ever be justified in an OSS operation?
I can think of one method. One sees it repeatedly in practice. Many commercial OSS companies have written their own code at their own expense. They cultivate outside developers if they can find them, but often they require that the outside contributors grant the company binary-only distribution rights to the contributed code. This enables the company to sell or allow distribution of special binary-only versions of its OSS.
This might take the form of allowing proprietary vendors to build products incorporating MySQL and distribute them as binary-only code (providing a barrier to competitors). Another variation is to have newer or premium versions of a software product that are binary-only; the OSS version always lags behind the proprietary version.
The benefit for the OSS company is that the binary-only sales generate the funds to keep the developers improving the product. There is far more need for skilled, unpaid developers than there are people willing to take these positions.
This was first published in July 2008