I'm not 100% sure where you're going with all that, but there are legitimate reasons for the super-user to "move down" into a specific user's identity -- for ex

I'm not 100% sure where you're going with all that, but there are legitimate reasons for the super-user to "move down" into a specific user's identity -- for example, to be sure that files/directories created during some maintenance are owned by that specific user, and have that user's umask applied. Another instance would be file cleanups when the super-user specifically didn't want to be able to delete arbitrary files. There are lots of other cases.