I imagine loading the top case to capacity with ball bearings would overload rack, and at a minimum take away some the stability of the bike.
While Samuel Clemens was famous for saying, "The problem with common sense is that it isn't common", I would guess most of us with top boxes find them the most practical piece of luggage for carriage, access, extra brake lights, less easily bumped, etc. And likewise, I pack my Dr. Pepper low in the panniers and reserve the top box for lighter things such as the potato chips.