I've never given it a thought. I only think of Folk Clubs in terms of membership. Nearly all of the clubs I have been to, that have guests, operate a membership system. I tend not to think of clubs that don't ever have guests as not being 'proper' folk clubs, more singarounds. I admit this is probably wrong of me.
At our club we have had a membership system for 42 yrs, so we can't be doing too much wrong.
For a very small outlay (a pound or two, i forget) you save £1 on the door price every time you go. So if you go to a club more than a couple of times a year, you save money.
Cheers
Les