On the rare occasion we use club sites at weekends we always book in advance to ensure a pitch.
This is where I think that priority should be given to those who are members. I know you can only book online as a member, but what about those who are booking over the phone.
I do know that on members only sites eg Incleboro Fields, West Runton you can nearly always get a pitch, out of school holidays, fairly easily, and do not always book when using this site, which will be when they first open and just before they close. At all other times we find it just too busy.
From a personal point of view, I think there should be more members only sites.