I own a car so it's not an issue of cost but convenience. Driving downtown is just not worth it (a broken window will ruin your weekend). I bought a car for leaving the city, not for going into the city.
I'd imagine renting the car is cheaper before you take into account parking, time, and potential theft. Car break-ins here are so bad people have become blasé about it. I went looking for a news story (there are many about how problematic property crimes are in SF) but found this instead:
