I found that the round trip cost of Mears was almost $100 for our family of five, and I booked an SUV for 9 days all taxes inclusive for $194, I thought it just made sense. We used our rental just about everyday we were there. With our portable navigation device I brought from home it was a breeze finding a destination and travelling to various places. And even though we stayed at the CR, I took the car to the parks sometimes (AK & MGM).
That said I certainly do not think that a car is needed, it seems that most do without it. Onsite tranportation is very safe and even fun for many. Many travelers even said that they had a vehicle with them and never used it.
Something to keep in mind is that if you decide mid trip you would like a car for a day or two, you can get Resort and/or Disney transportation to the CCC (car care center), and rent a car there for one or two days. You could even book this advance for pickup (or dropoff) at the CCC if you wanted to plan it that way. Another idea would be to book ahead and pickup at the airport for a couple or three days and return at the CCC. Or the opposite, pickup 2 or three days before departure at the CCC and return to MCO.
As far as getting to Universal, if it hasn't already been mentioned, you can take a taxi just about anywhere you need to go (this could add up to be more than the rental though).
Good luck with what ever you decide, I'm sure wou'll be fine with a car or without.