Glad to agree with you, Don!
If you want to see duos in performance in pubs, there are plenty of venues to choose from & acts to see. However, you won't be invited to perform, usually.
If you are a performer, and it seems to me that most audiences these days consist of performers, often of reasonably high standard, then the folk club offers a chance to everyone both do do their bit and to take in what everyone else is doing. Of course, there's an unwritten obligation for performers to make an effort to present fresh material. High standards- well, they arise from a shared bonhomie, I think. And it behoves us to be generous- we're all going to go for that top note and miss it one day! Sessions like the 8 Bells in Tenterden (2nd Tuesday, since you ask...) have seen so many great evenings when a mix of music has emerged.
The only possible exception to this, I think, is where a member of the audience accompanies a guest continuously, especially just ahead of his singing, and off-key. There I think the organiser has to diplomatically step in. And if it happens in your club, there really isn't any doubt about it!