well .. its a different philosophy ... I'm using Honda Diracc, and its a pretty good alternative to taking a cab. If you are using the car for the entire day, then obviously renting is better.
Sometimes its damned hard to get a cab in town, and with the peak hour charges, booking fees etc etc, car sharing is cheaper to use. I can just go to the nearest car port and pick up a car, and return it at any other port. My normal route is to pick up at millenia, (near my office) and return it at bedok (near my house) and its about $5 cheaper per trip compared to a cab during peak hours.