Tests have shown that it is quicker to drive to all parks except the MK. It can be close, but often the buses are quicker to MK. Of course, all of this depends if you get on the first bus, or the third/fourth bus because it is so busy. What you miss at MK is taking the monorail/ferry to the parking lot, then the tram to your car.
Another question is what is your next stop. It is not easy to get to DTD, and sometimes difficult to move between parks. We just like having our car at the end of the day so we can quickly get back to the hotel and get our feet up & chill. Signs are really good around Disney, but there is always a chance to get turned around.