From my experience with cab drivers (not Uber drivers), the idea is supposed to be that if you do not have a preferred route, the driver takes the most efficient route based on some kind of gestalt between lowest cost and fastest service.

The deal is, some passengers are convinced they know the best route from point A to point B, and will accuse drivers of trying to con them if they don't take that route. So if the passenger has a route they want to take, even if the route is stupid, you just shut up and do that. Otherwise, you take the best route.

In theory, of course. I have no doubt that there exist cab drivers and Uber drivers who run up the meter. Hopefully, Uber does some monitoring of routes and catches at least egregious cases.