' it goes with the territory of singing in public bars - but on the few occasions where I've seen what I've described happen I've always thought it a little mercenary to use singers in that way '

I can assure you musicians bestowed with the task of providing a few tunes at a singers' night aren't generally treated any different. Which is probably fair enough, singers who have concentrated on singing and listening to other singers need to relax, have a drink and a chat and what better time than when the musicianers provide an interlude. And ditto when the shoe is on the other foot.