Most touring acts that play in sheds have stereo systems. I know Dave Matthews Band was one exception although I heard that they have recently changed to a stereo rig. Obviously, the sign of a good engineer is someone who can use the benefits of stereo but also maintain even coverage to as many seats in the house as he can.
Many times amphitheaters use delay for fill speakers to cover the lawn or VIP areas. At big festivals, these delay fills are actually flown line arrays. Almost all of the time, these speakers are putting out a summed mono signal. As their name implies, the speaker's signal is delayed a few milliseconds to compensate for the distance between the listener and the main stacks. Ideally, they will both be heard at the exact same time with no phase effects or cancellation.