FWIW, when I stayed here at at the Westin for a few days, we used the car to get to the Kirstenbosch gardens and the Table Mountain cable car base station. Otherwise, it stayed in the hotel's garage as all the city centre attractions are easily acccessible without a car.

A car is useful, however, if you want to drive down through Camps Bay to the Cape itself, or take day trips to Stellenbosch or Hermanus.