Depends on if you intend on getting out of the city to explore the surrounding area frequently. I bought a car, but lived in Brighton and didn't have to pay for parking.

But regarding getting out and exploring, my parents live in southern New Hampshire, so to get to see them it meant walking to and waiting for a 57 bus, to go to Kenmore, to wait for the T, to get to park street, to then transfer to another train (or walk, in similar time) to South Station, to then wait for a Boston express bus, that would then drive to Nashua, to then get picked up by a family member to then drive to their house.

door to door could be 2 1/2 - 3 hours easily.

After I bought a car, that became a 35-40 minute door to door time. So if you plan on getting out of the city and value your time, keep the car.