Honestly, do you really think fans want to see other fans up on stage for a 2nd (or 3rd) time? I think not. They either want the opportunity to get on stage themselves or they enjoy the spontaneity of the moment. As Bono says, "Be in the Moment".
I am uncomfortable and disappointed in this contest. Why not give another huge fan a chance. There are many of us who have seen them in multiple times, in multiple cities in multiple countries. And not just on the U2ieTour.