Depends on what you want to do. Just go to the front and the back of the resort to get the bus to go to the parks? Then no. The internal buses are fine. Ride around and see the campground itself, then yes. But I wouldn't get one for the entire stay, just the day you want to do that.
When we stayed in the cabins with extended family (most inexperience, a few newbies), I was extremely worried about the transportation. But when my daughter-in-law, who had never been outside the state of Ohio and rarely leaves her hometown, had the buses figured out on the first day, I didn't even think about it anymore.