ZIpcar has a different set of problems; there is probably a car of the specific type you want available, but getting to it (and from it) may not be convenient, because it has a specific place in the city that it lives. ANd if that's 30 minutes away by your fastest non-car transportation, that's pretty frustrating.
I suppose for moving etc., you could call a cab to get you from old home to the Zipcar, and then again from the Zipcar parking to new home. That feels strange and probably-inefficient, but I don't have evidence to back up that feeling.
I can see how that would be frustrating. I guess my experience is not representative. The nearest Zipcar spot is a 2 minute bike ride from where I live. There's also the option of car2go, which seems to have a much larger coverage where I live, but also no variety in car choices. I'm not sure how much the variety matters, as I would use a car only if I need to transport something large (and I might just use a truck there; Home Depot is 15 minutes away) or if I was going a long distance beyond where public transit takes me (> 10 miles).
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.