I used to very much prefer BSD-style licenses on the grounds that freedom means you're free to create a proprietary derivative. I assumed that a company would rather contribute to a BSD'd project than a GPL'd project on these grounds.

It recently occurred to me, though, that some companies may prefer to contribute to GPL'd projects over BSD'd ones. When Red Hat contributes to the Linux kernel, they know that their competitors will not be able to create a derivative without also releasing it under the GPL. Red Hat's competitors can't create proprietary derivatives that would compete with Red Hat so, in essence, it's safer for a company to contribute to a GPL'd project than a BSD'd project.

This is a really interesting and non-obvious property of the GPL that I never see mentioned.