If you don't plan on leaving WDW property for something, then you don't need a car, but may want one for convenience.
We drive to WDW when we go, so always have our car with us. We probably use the car for half the trips, but also take Disney transportation quite often. The transportation is good enough that you don't have to have a car.
An awful lot will depend on how you're going to do the parks, etc. If you stay all the way past closing, then the bus lines can get quite long and in those cases a car can get you back a little quicker.
The one big exception of course is never take the car to MK. The bus goes directly to the park. If you drive yourself, first it's a long drive, then parking, then tram, then monorail/boat to the MK. You'll find that busses will be quicker than taking your car. One exception is if you plan to park hop, is go park the car somewhere, then use Disney transportation to hop. We've done this by parking the car at Epcot, but then not going in but taking the Epcot monorail to the TTC and then to MK. MK usually closes earlier than Epcot (at the time of year we normally go), so when MK closes, we monorail back to Epcot for a few more hours.