If ZipCar had a huge fleet where everyone who wanted one in Manhattan on the weekend could get one, that would be great... but then it would be sitting unused during the week, and be way too expensive.
This is the fundamental reason private ownership of vehicles is the mode, IMO. Everybody tends to use them at the same time.
One would one thing a constantly rental price could be used to balance these demand fluctuations. ZipCar could raise prices as the weekend approached just enough so a few cars would be left on the weekend for those willing to pay a serious premium.
The limit might be that most users would be very annoyed at the resulting uncertainty in how much they would pay for a car. A caveat to the efficient market hypothesis.
This is the fundamental reason private ownership of vehicles is the mode, IMO. Everybody tends to use them at the same time.