As long as you want to go downtown or Hollywood (and that would be a long ride from the airport area). I can't see taking the metro to:
Beverly Hills
Malibu
Santa Monica (easy cab ride though)
most of Pasadena
Newport, Laguna, etc.
Disneyland (unless you want a 3-4 hr metro to Amtrak to cab transfer)
the Getty
most of the Valley (not that it's a tourist destination)
Museum Row
...
Plus, how do you see two things in one day with metro? Not really feasible without a car.