With digital cameras and other low-cost person and location tracking technologies, it's not easy for drivers to take riders on any ride other than that which was contracted, nor for providers with bad reputations to borrow the identities of others. Branded quoted dispatching has returns to scale -- perhaps it's even a natural monopoly -- and incentives for self-policing. So bad actors can't just hop to another of multiple dispatchers and get the same amount of business.
Floating rates and open entry can restore service to areas that regulated rates sometimes cause to be unserved -- because they're comparatively hard to get to or dangerous, and there's plenty of business elsewhere.
And finally, for all the supposed protections provided by regulation, a city with strict regulations will also have a bunch of gypsy and counterfeit cabs -- and in SF, if local media reports here are to be believed, many of these are indistinguishable from the 'real' thing, even by most police/enforcers. You get in one -- and you get indistinguishable service as from other cabs, because the operators want to be cabbies, not robbers. And yet -- iPhone dispatching could give out a 1-time single-word-password more secure than any cab 'dress'. So if this were really something to be scared of, a branded iPhone dispatcher can provide more assuarance than the city medallion authority.
There is a valid concern at the heart of taxi licensing -- but it long ago grew into primarily a cartel, using that original justification to enrich incumbents, doing more damage than benefit. And, the restricted medaliion program is a solution from another era; let us try to get the same benefits with new tools, and see how it works.
