For 99.9% of what I'd use banners for, setting them at the usergroup level works just great. But there are times when it would be awesome to be able to accomplish a banner change on an individual basis as well (custom title).
This is a feature we'd love to see! Where users can custom their own banners without admins having to jump through hoops.
Also, where users can reorder their banners. That would be awesome - or to hide banners if they don't want them there.