Mike Olson commented that low cost distribution has been the key benefit of open source. He says that this can be achieved by other models and open source of the code is itself not key, merely a tactic.
I agree with Mike that access to read or modify the source is not the key benefit of open source. Most people rarely do this. According to Rishab Ghosh's FLOSS report many people aren't even aware that the technology they use is open source.
Question -"Do you use open source?"
Answer - "No, we use apache and linux."
However, a consequence of open source is freedom. With open sourced code, there is no market exclusivity and no lock-in to a vendor. Competition is on service alone, it creates a more perfect market.
With the future utility computing worlds of HaaS, FaaS and SaaS - where the need of a service is ubiquitous - this distinction is at the heart of the looming battles between monopoly providers and a marketplace.
So is the benefit of open source that the code is open .... absolutely, it's everything.