I actually interviewed at a company recently that brought in an Exec because the company had no direction and the founders were too inexperienced. (This is what the exec told me) It was really interesting to hear his perspective on the company and what needed to be done, compared to what others said during the interview.

But from that experience one person will have a lot of trouble guiding a company if the other execs are not on board with what they are trying to do. I was going to join the company but was really worried about the inexperience of the other team members, but I really liked the guy that the VCs brought in.