Gosh, I suspect there are lots of reasons these folks want to join a club. I can given some specific examples, but I suspect there are more out there, would be nice to hear from some of those folks.
One club I belong to has a good relationship with a toy store. The manager of the RC dept is a member of our club. He promotes the AMA (and makes his store some money), and gets a new member into our club (more members usually means good things right?). Win win so far. He takes care of the customers immediate needs, also benefits our club, and wouldn't you know, has a new customer that comes to him for future sales and service. That right there is called networking, requires that thing we've mentioned before called people skills, and really benefits all involved. Good news sometimes travels fast, and when that new customer/member talks to his friends, well guess what happens. Both entities enjoy the benefits. Remarkable eh? So along the concept of people skills, and news traveling fast, the club plays a vital role too. Is the club as you note, a bunch of 'ol farts flying a pattern with their Warbirds and J-3s (I added that last part). If that is what they are, and the membership is happy with that, fantastic. If they are losing members and wonder why nobody is joining anymore (and blaming kids and their lack of focus and they won't "build" and spend all day on computers) and complain about that...well that's another story. Or are they a club that opens it's arms to new members, and all disciplines of flying? I've visited clubs of both types. Guess which ones are more vibrant, and fun to fly at, and have no trouble keeping members, and have great events...etc etc etc.
Our club allows all types of flying. About 5 year back the old guard decided to boot out helis based on rumors and speculation, and even used supposed near misses and mid airs at other clubs as a reason to ban them. They played some games in doing so, but got exactly what they wanted, helis were out. Along with that went about 40 members. Oh, they were the ones who worked the events, always chipped in and helped out, did the cooking and parking and donations for raffle items. Poof....they were gone. That debacle was the reason I decided to get more involved and eventually run for office in the club, after being a noob to the hobby and the club of about 2 years. It took another two years, but after lots of hard work by many folks, the helis came back. They are a great addition to the club, and we were fortunate to bring them back, and give them their own flight line. MR of all types are welcome to fly there as well. Lots of folks in the club have fixed wing, helis, and quads. How silly would it be to tell them they can only fly 2 out of the three.
One big reason we have had at least 3 people who only flew MR join is the club itself, to have a place to go, have fun, learn how to fly and fly safely, and be part of a group that likes to have fun. There is a huge social component. Sure, some might get bored of flying there, but that could be said of anyone.
And just to be clear, clubs and their leadership have every right in the world to dictate their own destiny, to include or exclude whatever they want, as long as that is what the membership wants. If not, those that want change should step up and get involved, and work to effect that change they want. Or, join another club.
Oh, we fly gliders too! Because why not.