My guess about this one is that they don't want you to notice that you are almost always "logged in" into Google search if, for instance, you have a Youtube or Gmail account. With the fade-in, you don't really notice the "log out" option in the top-right corner.
I remember being very surprised to see that I was always searching in "authenticated" mode because I told Gmail to keep me logged in (btw, the option is checked by default so probably most users are).
I find it very frustrating that they decided to link all the accounts like this. I want to keep my search separated from my Youtube views/comments separated from my mail.
(of course: they can still deduce who you are without being officially authenticated, but that's another story)