Well, you probably heard of this, but RO tends to attract old players back somehow...
I agree it may be related to nostalgia--you want to play with friends once again, or that you played other games but always find RO to be the best in the end (due to the gameplay, way of community communication, etc). I find RO to be simpler and easier to handle than some of the other games out there (like Perfect World, which really has amazing graphics), and that's the style of games I enjoy playing (RO, Gunbound, etc).
I'm pretty sure private servers are worth it. They're worth the experience at least, to be a player, a GM, and/or an Administrator. And for those who want to recreate a server (not their first time managing the server), as long as you have the motivation to see your work through, then you should try it. It may be discouraging at first, but if you really improve the server and keep updating it, the fruits of your results will be yours for the taking (satisfcation mainly). The reason I became an Administrator before was for my friends, so I managed the server for the moment, experience, and memories that form. Even when I see a new player joining a server, I feel pretty content about it. "At least someone is trying out my work."
You'll only grow from here.