Great idea unless it's raining, this is why you see what's called "a roof" on most buildings......it keeps out 'most' things that fall down from the sky.
Indeed, for example at Bucharest ( Arena Nationala) to pull the roof of the stadium costs you 3,500 Euros and in 2013 ( Delta Machine Tour ) didn't rain a drop because was Summer and quite hot outside,the organizers pulled the roof because the sound to stay in the stadium. It wasn't Sold Out, unfortunately, but around 42-43.000 fans and the full capacity is around 50.000.
