Large banners really reduce the legibility of a forum. Most every banner I see here is large enough to be a nuisance, very much like driving along a highway and seeing one billboard after another - except that every second or third billboard is identical to a previous one.
Such things have rendered the SCIFI Channel forums, for example, unreadable and unusable.
At least advertising banners stay mainly in one position on a page - top or to the side. User banners are like turning most of the forum over to advertising.
I'd turn 'em off altogether.
That said, I don't expect that anything will change.