The big problem is that, no matter how small or inconpicuous it may seem, a PA takes on a life of it's own---and it there has to be someone listening from the audience perspective who has control of the sound levels--and it has to be someone with a good ear--

I was once in a band that played Scandinavian music, and we featured two fiddles who played in harmony--On one job, we played a stage outside with an independently booked "professional" sound set up, complete with a rock concert type sound technician--We carefully explained our set, did all the set up--about half way through the show, we discovered that the techie had decided that the harmony fiddle was the soloist and miked her up, and mixed the melody player into the background--