I have at least 3 different Apple IDs and it is a mess. I only ever wanted one. And I don’t know how I could have avoided it!
First I got an iPod and iTunes and created an ID. Then I bought an iMac and bought a MobileMe.com account, which created a different ID (I still don’t think there was a way to do this without generating a second ID). I also got AppleCare, and eventually ordered things through the online Apple Store (not the App store), and got an iPhone. At least one of these things created another ID. All these IDs use the same email address as my ID, except the MobileMe account. Finally iCloud came along and MobileMe (mostly) transitioned into that.
I didn’t even know there were multiple ones. The problem would be that Apple wouldn’t accept my ID and prompted me to create a new one, which I would do with the same Napier@Napier’sIsp.net address (with the exception of MobileMe -> iCloud). Then I would have weird behavior such as changing my password or other account detail and finding that sometimes the change appeared to have worked and sometimes it appeared not to. I eventually went hunting around and tracking down every version of ID I could think of that I might have, and at least making the passwords match.
But it is still a problem. Some of these IDs have security questions or other tricks associated with them, and I get prompted to authenticate and then get an error (because I did something wrong, I guess). Then I think one of my IDs is left in a questionable state and don’t even know which one it is.
I have heard that people set these things up deliberately so they can separate their home and work life, or to try to share music content with other people, or for other reasons. I don’t want to cheat the system AT ALL, and just wanted it simple and unambiguous.
What a mess! Apple seems to have steadfastly refused to merge multiple Apple IDs for people and won’t say why (though some time in September 2011 somebody high up in Apple seemed to indicate they were working on it, generating a spate of rumors that all these messed up customers were going to get taken care of).