ALL (not SOME, but ALL) major concerts, whether stadium or outdoor, are done in MONO. In small venues, stereo vs mono is going to be influenced by the size and shape of the audio environment. My references here are all about the audience, not the performers. With House systems, usually the 'house' will get individual mono signals from each performer, 'blend' them into a MONO signal out. Monitor signals are a different story and are usually custom configured for each performer on stage.

chas