That is if you're planning on being the car for an hour, and depending on the service, whether you're starting and ending near a parking spot. As in my example, if you're going to a hardware store and need a full sized car, ZipCar would be much better than Uber or even Car2Go because it's so cheap per hour.
ZipCar requires you to drop the car where you picked it up, right? So that $7-10/hour includes the time you spend on whatever non-driving activity you get up to at the destination.
Yes, as of 2012 when I could have used such a service. $10 / hour is a fine price for short-term car rental... but since I wanted to use the car to go places, as opposed to just driving around in the parking lot, it worked out to more like $50-100 per hour (of driving).
Bicycle rentals don't work like that -- you get on a bike at one stand and leave it at the stand closest to where you want to end up. I don't know why the car model is so badly broken.
Take a look at Car2Go, which has nominal rates a little higher than Zipcar, but pricing that scales down to minutes and is based around a "leave the car wherever" model.
Bicycles cost ~50x less but can be rented at similar rates. Also it's relatively easy for a bike-sharing company to redistribute bicycles on a truck if the supply bunches up somewhere.
An hour of Divvy is surprisingly close in cost to an hour of ZipCar, which is astonishing considering how much more it must cost to operate ZipCar.
Bikesharing is designed to encourage short trips within its service zone— even if you want a longer trip, you can just dock/undock a bike to avoid a fee (aka docksurfing). The escalating hourly fee is designed to keep the bikes in circulation, not as a charge you pay during ordinary usage— I've been a member of Citibike in NYC for over a year, and have never paid a fee beyond the membership.
With Zipcar, you first pay a membership nearly as expensive as bikesharing, then pay at least $10/hr.