Intrumentalists obviously aren't any more tolerant than singers. Singers without instruments have to stop and be an "audience" just like intrumentalists without voices do - it works both ways.

We've got a mixed session here that works pretty well, but the song to tune ratio varies with who shows up. We also do quite a few songs WITH accompaniment. I wouldn't do long acapella songs with no choruses though - that would try everyone's patience.