WHat about four speakers.......

When you are in a concert hall, sound hits you from all sides. Why arent there high end systems with four speakers; the listener would pretty much sit in the center, you get a box to delay the rear speakers ever so slightly to imitate those sound waves taking longer to reach your ears. What are the ups and downs of a second set of speakers behind you, even without any sort of delay? A few of you guys must have tried this, thanks......