Interesting! How do you usually plan your journey in Rome, and how often does your journey work out like the plan?
Consider that:
* I'm one of the few people in this city to be in walking distance from a subway station (blame it on archeology)
* Car sharing can be cost effective compared to public transportation: it costs 20-30 cents per minute, so if your trip is 10-15 minutes and you're traveling with another person the car is faster and cheaper.
* The only reason I consider the bus at all compared to the subway is because the stop is literally at my doorstep.