Each user has to weigh whether to use car sharing or car rental for their specific trip. Here's an example calculation:
http://www.carfreebtv.com/?p=273

H24/7 cars in most cities are mostly at staffed locations, so no, there's not much of a convenience benefit in those cities. In other cities, they're much closer, available 24h a day, are available without reservations or forms, include insurance, and available at somewhat predictable rates. That additional convenience, and the ability to only use the car for a single errand, may make it more economical. Or not.