On our first three trips (offsite) we didn't rent a car and relied on public transport/shuttles/cabs.
For every trip since then (all onsite) we have always rented a car for most, if not the entire duration, of our trip. Main reasons being: a) We don't want to spend precious time (and additional money) trying to get to the other places we want to go, like shopping malls, other theme parks, restaurants, etc. b) We prefer to eat offiste and, again, it is far more convenient to have a car than try to get where we want by bus or taxi c) After several visits, we like to see more than just the Orlando area so a car enables us to travel further afield and experience some of the interesting things that Florida has to offer.
I can honestly say that, after three trips staying offiste and using public transport, I woul dhave said that having a car was not necessary and I still agree. But it certainly does make life a lot easier for the way that we like to vacation.