As Ronda said, it really only matters if you're arriving late. In that case, you'd want to make sure they know you're comning so they don't go home thinking their job is done for the night.
Otherwise, the reservations pretty much serve to help them with their planning and dispatching. All that reservation info gives them an idea of how many people are arriving around what time and going to which resorts, and this helps them assign/dispatch buses as efficiently as possible. A few more or a few less won't mess them up and definitely won't mess you up.
On the way from MCO, you aren't assigned an exact bus. You can't miss your bus and you won't make the bus wait for you, which would delay and annoy others. So again, no worries.
However, on the way from your resort back to MCO, you are assigned an exact bus time, and if you miss that you not only risk getting to the airport too late, but also truly messing up the system by making a bus too crowded and not leaving room for people at another resort who were truly assigned to be on that bus.
