For a start off, let us not pretend that we play venues as large as that picture on any regular basis. Let us also not pretend that in small and even some medium sized rooms, the musician's own rigs (bass, guitar and drums, at least) contribute a large proportion of the sound that you hear, and the PA is used primarily for vocal reinforcement, and fill-in jobs. As such, it doesn't need to be in stereo for the audience to already hear stereo. After all, the musicians are spread quite widely.

And, what is the purpose of stereo? Let's think about that for a second...

Stereo was invented to allow those that were playing back music (initially, tape, then records, then radio) to hear the same effect as if they were in the hall, listening to the music. the sense that different players were in different physical locations, the ability to close your eyes and imagine that you were in the hall, listening to the performance.

Now step forward 50 years or more. We now have keyboards that essentially ARE playback devices for music (no matter whether our input is to a greater or lesser degree). And our audiences, although they THINK they are present at a 'live' performance, are almost in the same situation of listening to a recording. We play what we play on TOP of that recording.

Now, to increase the realism of the 'liveness' of that performance, using a stereo PA to add back those spacial cues that they would normally hear with a live band is quite effective. The real trick is not to exaggerate it... I rarely ever pan past ten to two o'clock, so those close to the left or right speakers don't miss an entire part, but the effects are ALWAYS wide. This can add a sense of spaciousness to the music in a small venue.

Another thing to take into account is NOT to space your speakers too widely. Most arranger default drumkits are panned fairly widely in the first place, and it is quite a job to go into every style you have and pan them back closer to the center, so placing the speakers no wider than they HAVE to be helps a lot, there.

I think the real trick is to not try and create a huge wide wall of sound, with you lost in the middle of it, but to keep them close, and just provide a virtual soundstage that recreates the stereo cues your audience would hear if you HAD a real band with you. You wouldn't want your bass player 30 feet away, or your drummer's kit to be 20 feet wide... Keep them in close, and keep things real.

I simply feel that mono, except for those few that ARE at the back of a big room or the extreme sides, is LESS realistic than well placed stereo.

JMO, yada yada yada....