We too have always flown and rented a vehicle. The only time we've used Disney Trans. is when we rode back from HDDR to WL parking lot. I keep thinking we should try the bus to MK but haven't done it yet.
The reasons we rent are the same as PP have mentioned, as well as others:
1. We like to be in control of our schedule.
2. We eat at alot of resort restaurants and they tell you to allow 1-1/2 hours if you are using the busses.
3. We like to explore other parts of Florida (actually my DM insists on this!)
4. We can shop for souvenirs and food (mostly breakfast and snacks) off site (We love Sam's Club and Super Walmart) I've even gone off-site to have my digital pix transferred to a disc (before I bought a bigger card) and had a relaxing pizza combo at Sam's, while waiting the hour. This was substantially cheaper than doing it at WDW. They wanted, I think, about $13 for 200 pictures. I got close to 400, at Sam's for less than $3.
5. We visit with my DM's uncle, who is in his eighties, and was at Pearl Harbor. He lives in Orlando.
6. We like to drop into DTD. With our own car, it's a quick trip.
7. We like to explore all the resorts.
8. We don't have to leave too early for the airport by taking ME. More time to say goodbye to WDW ('til next time!)
On a recent trip with 5 adults and 1 kid, we even rented 2 vehicles. This way part of our group could do something different. This worked well when my DM and DSis visited Cape Kennedy and the rest of us visited the parks. Also worked well when my DSis, and I, had a little blow up (we don't travel well together in a car-too close in age, I guess!
)
We currently have a standard SUV ,for 11 days from Alamo, for just under $260 (inc. all fees) We used the code from Walmart (#430710) for free additional driver and coupon from Entertainment for $25 off. Still hoping rates will drop further, or the rates on the minivans will finally come down. We've rented minivans, in the past, for about $225-250 for 10 days. I love Alamo because you get to pick your own vehicle. Everytime we've picked a mini van we've always been able to get one with all the bells and whistles (and built-in DVD player)
ADDED-I just checked Sam's Club and they are $367 for the same vehicle. Over $100 more. I've never found Sams to be very low on rentals.
HTH