We do it just to annoy you, and to be asked the same question repeatedly by users. It makes us happy..
To re-re-re-re-re-re-recap, the login in the header is part of the vBulletin package. The login in the Welcome block is a part of the vBAdvanced package, and the only way to "get rid of it" is to alter the core code, which means that with every single upgrade, we'd have to go back and re-alter it again, and again, and again.
This is not something we want to do, so stay logged in and it won't keep you awake at night bothering you..
Wayne