"Just works" is fine when it does.
A week or two ago, in order to try out iWork.com, I had to change the email address attached to my AppleID, because the old address (in Japan) is defunct now that we live in Washington, and iWork.com needed to send me a confirmation email. So I did.
Looked fine, until 3-4 days ago, when the App Store (or one of those quasi-cloudy Apple network things) challenged me to confirm my payment details (this was for free updates to free iOS software "bought" through that account). I cannot find any way to get past this challenge, no matter what variations I try in filling the form.
Sent email to iTunes support, got back a response, that, though pleasant and thoughtful, ultimately said "if you're not able to fix it without our further help, we won't either". Then they suggest PayPal accounts (don't want one, too much of my info is public already), or iTunes Store Gift Cards as the "solution". Have already spent about 4 hours on this, not looking forward to going through all of Support's suggestions. Something like this has happened 3 times in past, IIRC. Since I had no money at stake in past, I just abandoned the old account and created a new one.
But think about their "final solutions": if I can't get back into my account, I can't update either bought or free Apps. Probably can't reload any Purchased music I happen to mislay. Not sure if Home Sharing will stop working (I haven't really used it much), or Genius, etc. Can't update Xcode next time around. Probably can't Find My Iphone, and even if I can, may not be able to add new devices to the coven.
My thought on iCloud: there are two agendas, one for you and one for Apple.
Yours: keep all your stuff available to all your devices and safe "for all time".
Apple's: "Monetize" you.
If the agendas stop matching up well--as in it's too much work for Tech Support to restore your account, or Apple decides to drop some iCloud service and leave you hanging, or they start charging for something that used to be free, or charge more than before for something--they win, you lose. Probably no recourse.
Of course, none of that would ever happen.
Hmmm. Interesting issues you raise there.
On the username front, depending on which Apple service I use (I now can't keep track, tbh, so I try both), it kicks it back either when using firstname.lastname@example.org or email@example.com. A tad frustrating.