its hardly hacking the client. someone with some programming knowledge and manages to get hold of the server software could open a WoW server and bring out their own patches.
its common for any mmorpg to have private servers. last i checked they arent actually illegal, just against the developers terms of service, which, by playing on a private server, you dont actually agree to
private servers are ok if you are just looking for something to do, but for mmorpg addicts they cant really cut it, having no funding and minimum staff they can be unstable and 'lawless'.
back on the topic of this thread, i dont play world of warcraft, its one of the only mainstream mmos i havent tried
currently playing dark age of camelot (again)