A team entirely composed of E.coli experts will nonetheless will be slower than a team composed of physicist, biologists, etc in finding solutions because of the low diversity in ideas.

A programmer often have domain expertise in many area but they can still specialize by developing different domain knowledge, while retaining a wide range of knowledge.

A "business" guy, I presumed, not only know just how to run a business, but have domain knowledge in various area that a programmer would not normally know about?

So yeah, one guy can be on vacation, while another work and switch in between. However, you lose that creative edge and perspectives.

So there's no such thing as a free lunch in gaining redundancy in programming expertise.