It works well in DC because there is a good subway and bus system. As you say, it is used by people who need a car occasionally rather than daily. The people do not own these cars, the Zipcar company owns them and has them conveniently parked in many parts of the city to be picked up by arrangement.
So, when you’re pulled over by the cops and he sees a baggie in the backseat then you’re arrested even if it wasn’t yours. You’re also the one who probably gets the car that’s empty so you waste your rental time filling it up. Then there’s the problem with others doing who knows what in it. Or the person before you had the flu or smoked or stunk it up. No, thanks.