There should be a system preference to hide the login box on opac-main.pl, and that system preference should also make it so that the opacmainuserblock can be 12 if opacuserlogin == 1 but the new system preference == 0. I have a lot of people who hide it with CSS, but it means that a lot of front page real estate is wasted with whitespace.
Closing this due to age. This is manageable using CSS and Javascript.