I live well outside Boston but have lived in Cambridge. I'd mostly be fine with walking and taking transit around the city itself--which I've done. And probably mostly wouldn't want to drive. But I'd regularly want to get out of the city for hiking and other activities and I wouldn't want to deal with rentals every time. (People I know who do live in the city and do similar activities generally own cars.)

Even given a decent commuter rail system it doesn't help for those sorts of activities.

So it's not really getting around the city but getting anywhere outside the city that isn't on convenient mass transit.