There is something of a furore amongst composers that have worked for the Discovery channel, as DC has released a 5-CD set of library music but doesn't intend to share any licensing revenue (synch licences) with the composers.

www.filmmusicmag.com/?p=1760

This is not something that I am involved with much at all (fortunately). But it does highlight the danger of giving away too many rights in one context, as you might find them being used in another. To work with the Discovery channel, you must sign a work-for-hire agreement which many people feel is an abuse of a dominant position.

By the way, it's not clear until further down the article that the writers are still entitled to performance rights. But that in itself is a thorny issue given the poor tracking record of the US PROs.