The advantage for the ones who repeat the event is that they could have accumulated Stage lvl2 and lvl3 items and upgrade the Stage straightaway, not wasting time to collect itemsthey will need to or else no one will finish the event!
This is actually a very clever event because everyone is in the same boat no matter what level you are at. Everyone has one stage and everyone has time (whether you can use it to play the event or not) which means no one is at an advantage once the stage is built.