>Like what? Note that, for example, "They take up space on the road and pollute the air" doesn't count because not-for-hire cars do the exact same thing. It has to be an externality that is caused by the fact that the ride is for-hire. There may be such externalities, but I think they are far from obvious.

More specifically, the externality argument about taxis is that they take up space while hunting for fares, while private cars only drive when actually being used to get somewhere.

I agree though, that the point is irrelevant to Uber, since it's strictly a by-request car service that is not hailed from the street, and so doesn't cause the congestion that typical fare hunters would.