In case you can't tell from my picture, I'm a fan of the buses. To me, the buses are part of the Disney experience and the trip wouldn't feel complete without them. That said, it is a personal preference. My family has done both over the years, so here are my reasons.
Because there's a lot of people staying at the All Star resorts, buses come quite frequently. We usually stay there or at Pop Century (which has the same effect). During busy times (mornings, park closing), you probably won't ever wait more than fifteen minutes for a bus (our last trip, we never waited more than five during peak times). It can be a little frustrating waiting for buses, when if you had a car you could just hop in and leave right away, but some of my best Disney memories are from waiting for or riding the buses. We go as a family, so one of the main reasons we go is to spend the kind of quality (or quantity, depending on how you look at it) time together we don't get to at home. Waiting for a bus is one of the best times to do this. It's amazing how goofy you can get in just a few minutes in the bus lines.
At the end of the day, when you're exhausted, sweaty, and all you really want to do is sleep, Disney having buses seems like a miracle. I can't imagine driving from the park to the hotel after a full day; I would be too tired. Being able to hop on a bus is great. It's actually not uncommon for people (children and adults) to nap on the bus on the way back. Plus, Disney roads can be difficult to navigate if you're not familiar with them.
A few more trivial reasons: It is free to ride these buses also, so it's a good way to feel like you're getting your money's worth from paying for the resorts. Plus, it can renew your faith in humanity (I love seeing people give up their seats for the elderly, the very young, or the very exhausted :sohappy
.
And, it can be enjoyable if you get a fun bus driver (they sometimes share trivia, or useful information, or are just silly in general; we had one bus driver once who got the whole bus singing on the way to MK).
To me, it's just part of the Disney experience. I feel like the only real reason you might need a car is if you were planning to go off property (Magical Express gets you to/from the airport if you don't bring a car, and it runs very smoothly).