We don't have a car. We live in a city where we can walk most places (work, stores, parks, friends' houses, etc.). We frequently take the bus or the subway, but we almost never take a cab. My in-laws live only about a mile away from us, and they're planning to purchase a car seat for one of their cars which they will leave installed all the time. We do ride in their car or borrow their car sometimes, and we'll probably continue to do that once the baby arrives. We could easily borrow the car seat from them if we had a planned outing in some other car, although it seems like that is unlikely to happen very often since virtually all of our friends use public transportation too.
My worry had been about a medical emergency, but my wife pointed out that in a true emergency we're unlikely to take the time to install a car seat in a cab or a zipcar--we would either call an ambulance or my in-laws or hop in a cab with the babe in our arms. If it was less of an emergency (i.e. a sick child's doctor appointment) we would probably use my in-law's car regardless of if we had our own car seat or not.
Are we missing something here? We may well be--this will be our first child. What else should we be considering? Does it sound to you like we need a car seat?