Private Online games is ILLEGAL due to copyrights of the company and much more. Private Online Games today is a way also to play as whats good for the Real or Copyrighted Online Game. Just an example of Perfect World Game, you can try whats the best armors or weapons to the private server but if you already know whats good, you can implement your items to the real server. But sometimes, its really hard to play on copyrighted games because of the experience. So many Gamers play or prefer Private Online Games. But its a piracy. If you play private games just like Aion Online now, its release just months ago and then it comes the Private Server. Well as for me I don't have a money to spend that money to play this game, so I rather go Private Games.
If you wanna play Private server of Aion Online here is the link:
http://en.aionwars.com/

But remember when you play private: "You are playing a Pirated Game" its like downloading a illegal movies or songs from the net, you are the one killing also the Publisher for this game. Try also to think before you play private online games, besides if you don't want to play p2p games there are lots of f2p games that are available now. Or try to wait alternative for Aion Online but that is f2p just like Mabinogi:Heroes, C9, Tera and much more.