I rented a car in Cabo when I went for my honeymoon and it was very useful. We stayed at the Sheraton Hacienda del Mar and it was a ~10 km drive to town. We probably went to town almost every day to hang out in town, at the Office, at Playa Madano (sp?),...
You can typically get one at your hotel. That was my experience at the Hilton. We only had the car for our trip to Todos Santos. Otherwise, we took the Hilton bus everywhere. Check that, we also took a cab to get water and beer at a local grocery store. Dude didn't hide is porn comic books very well.
We figured that between the drive to/from the airport and the trip to Todos Santos that it would pay for itself and we would have it all week if needed.