Really no need unless you plan to head out of the city, and then just get a car for the day.

I may sound negative but there are too many reasons not to: parking (unless you're near a mall); traffic jams at rush hour; clueless, often dangerous driving; tolls; cowboy rental companies; cameras; 101 possible violations that you don't know about; road layouts constantly changing so sat-navs are out-of-date; etc, etc.

You need to be used to city driving on fast, multi-lane highways, as well as confident, careful, assertive and possessed of an even temperament !

On the upside, gas is cheap