I agree that it is hard to beat the convenience of having a car at WDW. In fact, we always used to rent one. We travel between resorts a lot and like to go places within WDW just for meals. We also like to visit the various WDW resorts just to walk around, see the decor. Of course, the biggest downside to WDW Transportation is resort to resort transporation. If you need to go resort to theme park, or theme park to theme park, the transportaion is fairly efficient, even though I hate using a bus. But if you need to get from the BC to the GF for a dinner reservation? That is a logistical exercise and a haul using WDW Transportation. Want to go from the BW to the WL to check out the decorations? Another tough trip. Point A to point B, transfer to another bus or boat, etc., etc. Not ideal. So it is great to have a car. Also, when your young ones are zonked out at the end of a park day, it's nice not to have to take sleeping kids on a bus. Yes, a car is very convenient, depending upon your party and your plans.
BUT.........
The last couple of trips we have done without the car. GASP! Didn't you just extoll the virtues of having a car? Why, yes I did. But things change. The prices for cars have gone up, and our kids have gotten older, and Disney offers free airport transportaion. So, we have done WDW without the car, and will be doing so again next week. Afterall, we can come up with a lot of alternatives for the $400 we would have spent on the rental.
Now we split stays. Part of the trip in an Epcot resort, part in the VWL. We do most of our Epcot and Studios touring from the BWV or BCV. Little need for wheels here when you have the boats, easy walks, and all those Epcot resort area restaurants in walking distance. Then we do the MK form the VWL. No need for wheels over here, again with boat transportation and all the great MK area restaurants. So that leave us with a bus ride to AK, and that isn't so bad. Having saved $400 on the rental minivan, if we have an odd trip here or there to get to a meal at a resort, we can spend the money on a cab. Even with that, we still save $200 or $300, and we don't have to worry about a car.
Having a car is great, but it has gotten expensive and isn't absolutely necessary, depending on your plans.