Most membership parks have a 2-or 3 week in one week out rule.. And yes when you are not there others may be using the site.. HOWEVER some parks you own the site. On those parks.. I can not tell you much.
Also most Membership parks have a mutual agreement with others for a small nighlly fee (I will use fifteen dollars a night as an EXAMPLE only) you can use other membership parks for 1-2- or 3 weeks at a time.
Advantages.. Well once I paid off my memberships (I have two of them) my monthly dues less than 100/month. In fact right now you can get a Thousand Trails Zone pass (One year of 2 weeks in 1 week out at about 1/5 of the TT/Encore parks) for about what Id pay for a week at some KOA's. Summers I use both memberships and full time for LESS THAN 100/MONTH!!!!!
Winters add another hundred cause of the week out park
Some parks are bright and shiny.. One of the parks I belong to the main park (Where i spend about half my summer) is a show place. Though they have one building in need of repair. the contract is signed and the work is starting.
The other one.. (TT/Encore) many of the parks are in need of improvements. which are being done.... SLOWLY. but I have to say I really like the staffers at the TT parks I have visited.
But the key to membership is WILL YOU USE IT.. Check out where the parks are. How likely you are to stay in 'em.. If you use 'em like I do they are great. Even every weekend you at least break even. but if you only visit for say 2 weeks a year.. Perhaps not the thing to buy into.