I think backing tracks could be a useful tool for a solo singer, but not at a folk concert or a folk club. Tim wrote "Seems a shame to bar someone from singing just because they don't have any one willing, able, available to play on the night." It didn't sound like the person in question was barred from singing, just advised by the organizer of the singaround that it wasn't appropriate to sing with a back up track in that particular venue.   Frankly I think that was a good piece of advice because I don't think most folk clubs would think it appropriate. It's entirely possible to sing unaccompanied (use a pitch pipe if you need a note to start on). As for PA systems, yes I think they have a place depending on the venue, the acoustics and size of the audience. You can still keep the sound natural while amplifying and improving the mix.