There's also the reverse of the idiot sound guy situation - the band idiot(s).
I went to watch a mate's band a few years ago and the venue had hired external PA with an engineer, pleasant guy who seemed to know what he was doing and had old but decent kit. I lost count of the amount of times he requested the guitars to be slightly less deafening so he could get a proper mix where everybody could be heard, eventually just shrugged his shoulders, gave up and did his best. All you could hear was drums and guitar.
Getting a controllable FoH mix is instantly torpedoed when someone decides they absolutely have to have their sound at the volume they choose at the expense of everything else, add in another band member with the same outlook and you can see why handing someone an XLR can be an appealing solution to multiple potential headaches.