I think that they always share buses except at busy times. They usually pick up from sports then music then movies and drop off in the same order. When we were there in September 2011 we shared buses to most parks but at night often had a bus for each hotel when leaving the parks.
We just left music. For most parks we had our own bus. Unless it was the middle of the day. Sports is first, music then movies. We most often got a seat. And most of the crowd was from movies. It really wasn't bad!
I was at Sports a few weeks ago and we often shared buses in the mornings on the way to the parks, but there were usually separate ones in the evenings on the way back.
Sports was picked up and dropped off first, but most of the people I saw getting on at Music and even Movies had available seats, unless there was an ECV on the bus or a lot of strollers.
We have stayed at Movies many times, we always have had our own bus for the past 5 years. I can only think of one time, many years ago when we shared a bus.