If I did not live so close to orlando (4hrs) and flew in, it would depend on how much disney I wanted.
In orlando there are numerous dinner shows, many small atractions worth visiting, and universal's two wonderfully put together parks. Wet-n-wild is the greatest water park, in my humble opinion. There is a lot in orlando you would be missing out in. Then you have Busch Gardens in Tampa, and Ybor in St. Pete.
Who can go to Florida and not visit the beach. Coco beach and Melbourne Beach are awesome on the east (atlantic). St. Pete beach is wonderful, and so is Clearwater on the West (gulf). Then you have some awesome state parks in the aera. You can go swimming is crystal clear springs, a cool 72 degrees. There is just so much. All of this is within 90 minutes of Orlando.
Put it this way, if you are going for on vacation at Disney, (and only disney) then no, I would not reat a car. I would use the moeny elseware. If I were going on vacation at Florida, and including a few days at disney, then yes, I would defenatley rent a car.