Back in the sixties when we started Cobbers, here in Australia, everyone sang all the choruses. If they didn't they were encouraged or abused until they did. It was an expected part of the show as well as being enjoyable. During the seventies, folk bands became more prevalent than solo performers and they tended to offer more polished presentations of the songs which made it feel a bit intrusive if you joined in. We were as guilty of this as anyone. It wasn't a conscious effort to exclude the audience, it just happened. One of the major factors (in my opinion) was that the songs became faster and I've experimented a bit with this over the last few years. My observation is that if you want people to sing along with you, slow down so thay can keep up. As for those who say they can't sing, well neither can a lot of people who make their living by it. When we were recording, there were often people who couldn't hold a tune particularly well in the band but when you added them into the mix they gave the choruses a richness of texture that wasn't there without them. In some songs where we were trying to emulate a bunch of shearers around a camp fire or something similar this could be really brought up and fitted well but even in songs which were more melodic, so to speak, those extra voices added something special to the sound.