The age old last mile problem. I don't think anyone is suggesting getting rid of mass transit and in fact self driving cars could be the best thing ever happen to mass transit (no need to park at the train station if coming from the suburbs).
Having the commit will make it maore feasible to fund vehicles that are intended for a specific area, and the pool model allows Uber to recover margin during pricing surges and users who don't want to pay the commit (after all, the discount on the fare would only apply to the portion of the pool fare ascribed to the subscribed to the passenger).