There's no proprietary attitude inviting programmers to keep their sources to themselves in the hopes of making a ton Collaboration ensures that problems are solved fast. There's no calling Tech Support to wait on the phone for hours with Linux. The answers are on the Internet, and in the minds of the other users. Open code means that anyone might solve your problem. It means that your organization might solve its own problems.
Open code ensures that bugs are found quickly, and fixed quickly.
The community shares its knowledge. With Linux, there is no waiting years for the next hyped release, for which you shell out even more buckets of cash. Next, Knowledge and it's pursuit are motivation enough for many people to install Linux. Linux knowledge is vested in individuals, instead of being handed over to a large corporate entity. Anyone with the proper motivation can become an expert.