We almost always rent a car when we go to WDW, even staying onsite, and when we don't rent a car I invariably come to the "What was I thinking" viewpoint. Here are the reasons we rent a vehicle:
>No waiting in long lines at the end of the day. We just shuttle to our parking area and head to the hotel. We've timed it several times (me in the car vs. sister on the bus) and I always get back to the room before her, sometimes with enough time to have showered and been in bed!
>When staying at a Value resort, we can park very, very close to our room. Makes it convenient to do an early morning coffee run to the food court if your feet are blistered and sore from the day before. Also makes it SO much easier to get back to the room at night, especially with sleeping/sleepy children who might need to be carried.
>Buses at park closing are often packed. We've never waited in line for less than 30mins. to get a bus back to our hotel. There are also a minimal number of seats on the bus, so after being in the parks all day and waiting in line for what feels like forever, we have the pleasure of standing on the bus, packed in like sardines. There seems to be no maximum number of standees on these buses. It's hot, crowded, tiring, sometimes smelly. Now think of trying to stand on this bus while holding a sleeping child. No, people will not automatically give you their seat if you are holding a sleeping kid. Many think, well, I'm tired too, and I got the seat first.
>People talk about parking at MK like having to take the monorail or ferry across 7 Seas Lagoon is a bad experience. Seriously? Where else can you ride a MONORAIL? One that goes through a building? How often do you get to ride in a boat? We think the monorail and ferry are part of the fun, two of the attractions, of going to MK!
>We like to have dinner at restaurants located in the resort hotels ((Boma, Jiko, California Grill, Whispering Canyon Cafe, etc.). Unless you are in the MK and eating at a monorail resort, we find it easier and faster to just use a car. One memorable evening (and our anniversary), it took over 2 hours to go from POFQ to the Polynesian for dinner at Ohana. We'd left our hotel room 2 hours before the ADR time and we were late. They'd given our table away, but would "find" one for us if we were willing to wait. What were we going to do? Took about 45 mins. to be seated. If we'd had a car, we could have been there in about 15 minutes, with another 5 to walk in from the parking lot.
>We like to go to the grocery store and get in-room breakfast items, snacks, wine/beer, etc. It's much more affordable to just stop at Publix than to use a grocery delivery service. A car also gives us the ability to check out towns like Celebration, go to the Disney outlet stores for souvenirs, eat dinner off-site, etc.