I think I understand what you are saying.
I don't see a great deal of value in the fixed schedule either but I see some benefits.
Maybe scheduling a show that ends just prior to the scheduled bus departure will work but for the rest, the planning will have many variations. We always manage to "throw away time" when going to a restaurant reservation but at least there is some flex there.
If we get off of a ride and only have 20 minutes before the next express bus departs, we are not likely to fit anything else into the schedule and will end up waiting. The waiting could entail some shopping or looking around the park (e.g. trails in Africa at the AK) which would all be better than standing or sitting in line at the bus terminal outside of the park.