I have no problem with booked acts who are multi-instrumentalists who then use their talents to the full. As long as they stick to their time on the bill! The problem is generally with the floor singers or people on an open mic night who come for a 10 or 15 minute set with all of their instruments and expect to use them all. I've seen bands show up for showcase spots (not at AFO!) with more instruments than the booked guest band.   I'm always puzzled about what kind of impression they think they've made when they overrun their allotted time. I remember working as a duo in the past and consciously keeping any short spot simple and direct, with minimal fuss.
But oh, yes, the sound check problem! I've lost count of how often my own sound check has been cut out completely or I've found myself sound checking in front of the incoming audience because of a band who had to check every combination of instrument and voice and threw in a bit of rehearsal as well ....

Anne