Did the "perspex screen in front of the drums" question ever get answered?
2 main reasons
1) The weather - strong swirling winds and heavy rain can play havoc with a drum set. Obviously there is a roof but as U2 found out if the weather gets bad there then the stage gets soaked anyway
2) Some drummers like the sound they are playing to reverberate back, it helps them keep rhythm. (Especially with such a large stage like that the sound goes out and they can barely hear what they are playing.)