It seems that an element of defensiveness has crept into this discussion.
Guest, Val - your point about audiences being "desensitized" is well made, and yes, clubs can easily make their own rules regarding instruments or no instruments for a particular theme night or whatever, or experiment just to test the water, that doesn't mean it has to be a permanent change in whatever direction.

Villan - no need to get yr coat, no criticism was intended of any particular club, I and other correspondents were just making observations on trends in and club attitudes in general, in my humble case based on experience of "all three sides of the fence" (and in different geographical areas, though not the States or Canada I'll admit) over a period of some 25-30 years.

>>Why must a club cater for both.<<
I don't think that's what folks here are saying. It's not a case of "must". I'm sure nobody's trying to tell anyone how to run "their" club. Many of us are currently, or have at some point been, in the position of getting a club started and/or keeping it running, and all manner of factors come into play, which may vary from week to week. As Villan rightly says, >>At the end of the day its up to each club to decide what they want and stick to it. It is also up to the audience to decide where they want to go to.<< Yes, but even a "loyal" audience can be notoriously fickle, and you might well get a full house for Mr Carthy, say, if he was to advertise he would do a whole evening unaccompanied!

>>I have always tried to be honest with performers and if their style suited my club, great, if not then I tell them. That doesn't make them bad performers.<<
Both points particularly well made there. Honesty from and among club organisers will always be best policy for the performer, and for the club's audience too (if that can be accurately gauged). Again, the last sentence is the key, and only bears out what we said earlier.

Finally: Scrump - your >> "It's true that an instrument helps the singer to stay in the right key throughout the song" is only true if one is able to tune and play the instrument in question<<
Yes, and that applies too if the instrument in question is the voice itself doesn't it? There's no disgrace in an unaccompanied singer taking his/her pitch from (say) a concertina before starting the song, and neither does that need make him/her a "bad performer", any more than it makes an accompanied singer who's reliant on an instrument a "bad performer".