One of those dimensions may be service availability to low income neighborhoods. Cabs are supposed to take fares anywhere
Just because they are supposed to doesn't mean they do. The first few paragraphs of this article[1] details one journalists' attempts to get taxi service in DC. When you force entities in an industry to subsidize some services below cost (such as rides out to the boonies), you don't always get the service below cost; sometimes the entities "forget" to pick up the fare.
(I don't mean to suggest that Uber is the only way of solving that problem.)
Just because they are supposed to doesn't mean they do. The first few paragraphs of this article[1] details one journalists' attempts to get taxi service in DC. When you force entities in an industry to subsidize some services below cost (such as rides out to the boonies), you don't always get the service below cost; sometimes the entities "forget" to pick up the fare.
(I don't mean to suggest that Uber is the only way of solving that problem.)
[1] http://www.theatlantic.com/magazine/archive/2012/05/why-you-... and the writer is an Uber fan so apply some grains of salt.