There is no transferring needed with Disney buses.
Except if you want to go from resort to resort (e.g., the Hoop-de-Do). And if you're at most of the MK resorts, you're going to have to take a monorail (after having to go through security) to get to the TTC, and then take a bus to go to a different park.
They drop you off closer to the parks than any parking spot. When you drive you will most likely either have a long walk to the park gate, or have to wait and take a tram in.
No doubt the bus drops you off closer than a car. I think we had one "long" walk from our car to the gate; the rest were all <5 mins. And the trams ran pretty frequently (although we found walking was often just as fast).
Also, if the walk or tram is going to be a problem, you can always Uber instead of renting a car. That will get you just as close as the bus.
Off site is also a hassle if you think your family will want to split up for part of the day, or make trips back and forth to the parks.
I concede your point about splitting up (unless you're ubering--but then you're paying double fare). I don't think any of our car rides took more than 15 mins door-to-door, so unless your bus route is faster than that, making mutiple trips makes driving an even better option.
I'm not going to attempt to say that an off-site bus is convenient.
Driving--I have done this many times too. If you are off site you have to pay to park. And then park where directed. If you are super lucky you can walk in (hope you enjoy your extra stroll) or you can wait for a shuttle to bring you in. You may have to wait for 2 or 3 until you can actually get on one. And then do the reverse at night. Only now you are tired and have to fight the traffic yourself, instead of having someone drive you.
No denying the parking charges (unless you uber, but then you're paying that instead). Again, I found the walk between the car and the gate to be de minimis, esp since you're walking all day anyway.
I was SHOCKED at how fast and easy it was to drive out of the parking lot at the end of the day. I was expecting stadium-style gridlock, and it was nothing like that at all. (My sports teams could learn a lot from Disney's efficiency.) I'm telling you, I don't think we ever had more than a 15-min car ride, including traffic.
I'll grant that if driving home is going to be tiring for you at the end of the day (or you want to drink), it's not an ideal option. Though again, you can uber instead.
I've been talking about uber a lot. We decided to rent a car instead of ubering mainly because we had little kids and didn't want to have to deal with car seats in an uber. I think using uber would be slightly more convenient than driving, but there may be times when you have a slight wait for a pickup.