Don't forget that if you're parking in the parking lots, you can still have a long hike to get to the park entrance, or you have to take a tram from the lot to the entrance. This defeats some of the self-driving advantage.
And for MK, you have to take transportation (water ferry or monorail) from the parking lot to the entrance, so that again nearly defeats the entire purpose of driving yourself.
Personally I would hate to drive and park myself. It seems like unnecessary extra "work." The beauty of Disney, for me, is that you're not forced to do "real world" things yourself. Driving and parking in parking lots are "real world" hassles to me. I do them every day. It's not fun. Worrying about traffic, finding a parking spot... things I'd rather not deal with. Let me hop on the bus, sit back and relax, then magically arrive at the park without a care (where did we park? will there be traffic? Will the hotel lot be packed and we'll have to park farther away from our building?)
The only negatives about the buses is that occasionally the bus stops are full and you have only standing room left on the bus (this is usually only after park closing). Standing for 20+ minutes on the bus at the end of a long day is horrible. If a bus is standing room only you could always wait for the next one, but that does mean you'll have to wait a little.
But ultimately I prefer having Disney take care of me, not having to "think": where I'm going, what turns to take, writing down where I parked the car, waiting for traffic, paying attention to the road, being a diligent driver in traffic. And if you want to take a mid day break, you have to pile into a hot car, wait for the AC to kick in... then go through that same process when you want to go back to the parks.
Just let me get on the bus. Let them take me there.
Point being is that it's not always cut and dry about which is "better." If you'd rather not wait for a seat on a crowded bus, driving yourself might be for you. And you might get to some parks a little quicker by not having to wait for a bus. But the mental rest of not having to drive, park, drive, park, etc is worth a few extra minutes of waiting for me. Being mentally fatigued is just as bad as being physically fatigued, and driving at the end of a long, tiring day isn't fun when you'd rather get on the bus, let them turn off the lights, and lean back and close your eyes while they take care of you.