I agree that the GPL is vital for getting contributions from commercial entities. In my company, management reacts very strongly to the possibility of giving another company an advantage and getting nothing in return. If I release code for a BSD-licensed project, my company might be subsidizing competitors, other companies that use our code but don't release theirs. The GPL provides a legal guarantee that our competitors have to play fair.

The legal guarantee is actually all we need. I'm not sure why that carries any weight with the bunch of ruthless mother...s I work for, but there you have it.