(1) The buses don't always arrive every 20 minutes. I've waited in excess of 40 minutes for a bus...and not as a result of crowding and having to wait for other buses, either.Why do we need to track the shuttle busses? I'm ok with knowing every 20 minutes (or so) the bus will be there.
(2) Personally, I'd rather NOT arrive only 1 or 2 minutes AFTER a bus has LEFT, thereby not knowing I have (at least) 18 minutes until the bus arrives.
(3) In the mornings it probably doesn't matter so much, since buses are more frequent, but I've often waited for a bus to a particular park, and in the 30 minutes I have to wait, I see 4 buses (empty) come and go to Downtown Disney. If I knew I'd what time the bus was coming, I wouldn't have to stand in the hot sun for 30 minutes, but can find some shade.