I think the best benefit (other than express pass) is being able to make sure everyone in your party can have a great time. Nobody is inconvenienced.
If you stay onsite and sister Jackie is taking 2 hours to primp in front of the mirror, you can leave her in the room and go to the parks. She can catch up with you later by taking the bus, walking, using rickshaw or the boat. If little Mikey is feeling sick in the park, one person can take him back to the room and everyone else can stay in the parks. There is no issue of EVERYONE having to go back and forth to the parks at the same time because you only have one car.