Of course, it can be suggested that many performers feel reluctant to voice their opinions in song or otherwise because of type of scenario to which Jim has described... i.e. they may think that it might affect their career(s) in that they will receive fewer bookings or deter audiences in general. However, that's a decision for them and it depends how true they wish to be to themselves.

I don't particularly like the idea of "Singers only" clubs or "no instruments, please" scenarios but I'm not in favour of entirely instrumental arrangements either although I appreciate that these do occur. It's only natural that people with similar interests will gravitate towards each other but I prefer it to happen on a "de facto" basis.
There are obviously events primarily intended for the benefits of singers and others for musicians and all manner of things in between. However, most of us know whether we should be there or or not and what is usually expected. For example, we know that a singaround is not the same thing as a pipe band or a Strathspey and Reel Society!

Most folk clubs will generally tend to cater for their regulars and members whatever their tastes may be but there's not necessarily rules set down in stone.
Our own local folk club actually books a wide variety of acts ranging from unaccompanied singers, to topical singer songwriters, traditional(Some purely instrumental) bands, blues artists, guitar virtuosos etc. Just about every area at some stage or other. Fortunately, we are in a largish catchment area and it doesn't matter if we don't attract exactly the same audience each week. Some nights are obviously busier than others but we usually "break even" at the end of each year. However, there is a good argument that we could be "more focused", in which case we would encourage a more loyal and regular audience. However, this would mean narrowing our musical output.