There is one thing everyone is forgetting, 99% of the audience can see, and therefore providing there is a wall of Mono sound, the brain will take its positioning cues from the eyes, however if a blind person attends, then all they hear is a wall of sound, as there are No aural cues as to where the individual instruments are.
With an Arranger, All your audience are essentially blind, (There are no visual cues for the brain to assimilate) and therefore to give the impression of a live band the ear/brain needs the stereo effect to get the aural cues.
BTW if you have real brass instruments on the left and real strings on the right, then as you walk to the left the strings will get quieter, and vice versa, consequently unless you have a stereo speaker system, arranger playing just doesn’t sound real.
Another way to look at things

Bill