I felt it was time to resurrect this interesting thread. I wish I could remember where i read it, but ELP supposedly put another speaker horn in the middle of the ceiling of Madison Square Garden during the 1974 tour, and threw effects on top of the crowd as well.
Roger Waters gave an interview where he was explaining the diamond pattern of Pink Floyd's quad setup. Music comes from the stage, both sides, and one set of speakers from the back, as someone mentioned earlier. He said people hear music in front of, to the sides, and behind them. Not from front and back left and right corners. Makes sense to me.
When I saw the 2006 Dark Side tour, Waters had cones in Madison Square Garden that shot the sound in different directions. This worked great if you were on the floor, which is what I noticed at his tour of The Wall in 2010 in another indoor arena in Houston. In 2008, he played an outdoor amphitheater shed, with the diamond arrangement of speakers surrounding seating area where I sat, and another one for the lawn seats. It worked better than the cones in 2006 since at that time I was sitting in the back middle rows. The effects simply got louder rather than creeping behind me. But when I sat in front of the mixing board for The Wall shows, the cones worked very well, and it was stunning.