I think of Bob as a team communication tool. Hopefully, most people on the team are familiar with Bob. When I'm working with someone else on that critical thinking thing about the end product, I can ask "What about Bob?" and teammates know what I'm talking about. Bob is a mobile user might not be complex enough to warrant a persona. I think personas are more likely to be a useful tool as the number of user types/goals increases.

Personas can also be a kind of checklist as a change is made, ensuring that the change makes sense (and doesn't break) in the most common product scenarios.