"Guest" has unwittingly reminded me of another point...

Know your audience or potential regular audience and, within reason, cater for their tastes and wishes.

If all your regulars are budding singer songwriters or fans of this type of thing, you may indeed "kill the club" if you book the traddies. Of course, the opposite scenario could equally apply.

Likewise some people enjoy or prefer singing(Even eschewing any form of accompaniment) while others prefer instrumental music or a mixture of the two.
In some clubs, it might be more of a social night out with "locals" rather than "folkies" and they just want to be entertained.
In certain areas, "Americana Music" seems to rule and some clubs will always prefer to book visit American acts before anything from these islands.

Of course, it's up to the organisers of the club to decide on the music programme but, ideally, there should be a demand for what they are doing. Or the potential to create a demand....

Our own folk club (EFC) has a much wider variety of music than most others in the area but we get away with this because we are in a large catchment area. We do actually have a large membership too but, unlike some of the smaller rural clubs, they don't all come every week.
So, while we have fewer *regular* attendees than some other clubs, there is usually the option to attract visiting punters as long as its somebody (or group) they really wish to see.