We always rent a car. Several reasons why..
We stop on our way to our resort at
Walmart or Target and get things for our stay. Breakfast in the room, snacks, etc. Even when on
DDP.
We don't spend all our time at WDW. My DM, who comes with us every time, insists that we check out other parts of Florida. Some of our best times have been at Florida beaches. DD11 would spend all day, every day, at the beaches if she could.
Eating at other resorts. Getting there by bus can be very time consuming. I can leave POP and be at Kona Cafe in under 30 minutes. That's counting parking and walking in.
Stroller-when we had a stroller (I miss those days) DD could ride all the way out the park to the car. If she was tired, I didn't have to carry her.
Park Hopping-we like to park hop. Getting from MK to Epcot is easy. Other ones can take more time. Last year we left the car at DHS and went to Epcot by boat. It took quite a bit of time and we were lucky we made the boat. Otherwise we would have to wait another 15-20 min.
Speed-I can get to one park from another park much faster with my car than they can get me their by bus. Last year DD and I were able to get to AK from DHS, and inside the park, to ride EE 5 times before the park closed, in less than an hour. Drove, parked (up close), enter park, and quick walked to EE.
Eating/Shopping off site. Saves $$
We buy most of our souvenirs off site. Target and Walmart have a great selection of Disney themed merchandise (though honestly we aren't big souvenir shoppers).
Eating-if you aren't on the DDP, eating off property can save you big $. For example the McD's located on fairly close to AK has the regular menu. Inc. the $1 one. The one on property doesn't.
Price-most of the time I can get a pretty good deal. Last year I spent $204 complete for 10 days at National. And we got a really cool car.
I actually never took a Disney bus until a few years ago. My DD and I stayed at DQ at DTD while my DM took the car back to POP. It seemed like our bus was the last one to get there and even though it was the middle of the day there weren't enough seats and I stood all the way to POP. I would say we waited a good 20 min. for the bus.
Last year we decided to use the bus to go to MK because I heard it was so much easier. Granted, getting to MK from POP was easy. But getting from MK to POP was not. Our stop was the last one down the line. Buses for all the other resorts came and went and we were still waiting. When it finally arrived the driver had to help load the wheelchairs waiting. I don't know if he was a new driver or what, but it took him a really long time to do it. Somewhere between 5-10 min. while we all stood there waiting to load and watched more buses come and go. And again, the bus was packed and we stood all the way back. We won't do it again. It's the car for us.
Oh, and one thing I've started doing that has worked well for us....
Before we leave our resort I put ice in an ice chest (picked up at WM) and bottles of water and soda. Then when we get in the car we have a nice cold beverage waiting for us! So refreshing, especially if you are leaving a park in the middle of the day. If you are on the dining plan, get bottle beverages as your drink choice and use those.
So that's my 2 cents.