Open source is better engineered than closed source. Chris notes that while closed source environments tend to be better funded, there's a strong possibility that they were created in isolation without the benefit of a large and robust community to help guide the development.
Open source, by its very nature, builds in more flexibility and adaptability which is necessary for the more complex systems of today. We're now dealing with a broad range of infrastructure architectures, delivery models and methods, data access levels and formats, so "everything must be integrated now" ... adaptability is key. Closed and open must co-exist.
Open source is less dependent on a single vendor. (See #2 on the need to be integratable.)
Open source is easier to transition with. The tools that make up an open solution are "knowable quantities." You won't need to reverse-engineer an open source app or tool if the developer source disappears. It keeps you from being trapped in a bad solution.
Chris reminds us that the main message is that regardless of whether your technology is open or closed, it has to be able to integrate.