I live in brooklyn and we bought a car just for errands/grocery trips/and weekend getaways/vacations.
I'll still bike to get dry cleaning and even walk to the grocery store (its only two blocks away) but sometimes you go to BJ/costos/trader joes/ikea and the car becomes a god send.
Car rental rates went up twice this year and zipcar is nice but even more expensive.
my rule is if its with-in 2miles then I'll take the bike.
I think the car has been super useful bike wise - in the sense that where ever we go (like a quick weekend trip to Montreal) we can bring the bikes and REALLY explore a city.
Its great to do things like pick family/friends up with all their luggage from the airport or making a pizza run when its raining cats and dogs....
but I still get pangs of envy when I see people zip by me on the bike lanes especially when I notice that they can cross the whole williamsburg bridge faster than me in a car sitting in traffic.