Comment 38 for bug 844081

I'm thinking of it like this: the background picture in 12.04 is a personal
touch in the same sense as your name or your profile picture. It is part of
your public profile, and it's a way for the OS to say “this belongs to Bob.”
(It works like this in lots of popular web services, for what it's worth.
Facebook, Twitter and Tumblr spring to mind).

That IS a change in approach from 11.10, where the background was visible
by circumstance but not necessarily on purpose. They're different
approaches, and that's pretty well it. Ubuntu's backgrounds are not like
other desktop operating systems, but neither is Ubuntu. The feature makes
sense in its own right, and it's quite clear - from the first time someone
logs in - that it is happening. I, for one, don't see a problem there.