Surely it not that difficult to make one's preferences clear, ie, "No accompaniment on this one lads, thanks", or whatever, and most musos will get the message pretty quickly. Some musicians feel obliged to accompany singers and it can be a relief to just relax and listen. It's like that at most of the sessions I go (practically all mixed tunes/singing). The only disturbing thing might be folks joining in the chorus, but surely even unaccompanied singers can live with that?