We have let members of the audience sing but ONLY if we know them, and then very rarely.
One night as a favour we let the brother of a regular get up and we could not get him off. He turned out to be a really obnoxious individual with a superior attitude, a real jerk.
He was on holiday from England and upset some of the canadian audience when he announced " here is a song by John Denver, I suppose you have heard of him over here in the colonies. He was not trying to be funny either he was quite serious.". Anyway after 2 songs he refused to give up the mike and was settling in for the long haul when we unplugged the system, he got quite indignant and to add insult to injury he was f....ing hopeless, off key, no quality to his voice and did not know the words and he was not even drunk.
Better to let the people who are getting paid for the gig do the gig.