The only time I've had to wait for a second or third bus to show up was at a park closing (or after fireworks/parade in the case of MK, when the park stays open beyond that time).
While "bendy" busses would be more efficient in that specific case, they've got to eat a bunch more fuel than standard busses. And since the only time they'd fill them is at park closing, I don't see how in a cost-benefit sense, they'd work, unless they sat at the parks and didn't leave until nearly full - and I sure as heck don't want to be sitting around waiting for a bus that size to fill during the middle of the day when I just want to go back to the hotel for a swim...