In response to:

"why it was often necessary to use PA in a venue small enough that you ought to be able to do it purely acousticly. But in that thread, everyone seemed to think that a good musician or group would have no trouble getting everyone to be quiet and listen, so if we needed PA we were probably not that good."

I think the critical factors are

a) the size and acoustic of the room - even a silent audience soaks up sound, making amplification necessary if there are more than say 50 people (and many folk fans are not in the first flush so hearing may be an issue).

b) whether they've paid to get in or not. Free entry often means some people are not actually interested in the music, only in chatting and drinking, so playing quietly won't work. The 'soft trick' does usually work if people have paid to get in, and if the talkers don't notice and shut up (they often don't) the audience often shush them for you. But only if they really want to hear you (money at the door being the best divider I've encountered so far).

PS I think the deal worked because my lad likes to listen now and needs no bribing. But he's not interested in playing because he's not musical. (Well, it happens)!