The whole point of stereo is NOT that everybody in the house gets to sit in one 'sweet spot'. That's absurd. But equally absurd is the contention that ONLY those sitting in the 'sweet spot' get ANY stereo experience. I am SURE that you don't run your home stereo in mono unless you are sitting perfectly at the apex of an equilateral triangle with the speakers at the other two corners. It doesn't collapse to mono the minute you move your head or body!

The strength of stereo is it helps localize certain sounds, and give a sense of dimensionality to the music, and helps avoid different parts from clashing with each other. It helps recreate the actual stereo sound of a live band (you don't need a stereo PA to know and hear the bassist is on the left and the guitarist is on the right in anything other than very large concert venues). And recreating the 'live band' experience is EXACTLY what arrangers are all about. That includes positional cues, as well as musical ones.

Sure, there are the occasional venues, as described, where stereo does NOT work. That is what the 'mono out' jack is for, on your arranger (well, some of them )! But in the large percentage where stereo DOES help you get a sense of dimensionality into what is essentially a 'fake' performance (there is only you, not the whole band the customer is hearing), it is a plus, at least as important as a reasonably 'flat' sound...

I think most of the objections to running stereo seem to come from those with theoretical reasons for it. But actually USE a stereo PA, and you will quickly come to see that, for all but the very largest gigs, it IS apparent throughout the venue. Not as good as the 'sweet spot', but still there...