Although I'm not in possession of a copy of MAC, I'd say that if a martial art is training people at rank 2 with weaponry than at rank 4 they still teach people how to use the weapons next to the rank 4 techniques... I see no problem in allowing that. But if the martial artist is not progressing, than it makes sense to limit the max result on the weapon table as well.
For me, weapon katas don't really add to the result of the martial arts. I'm allowing weapon katas only to attack those heavily armored enemies with strikes and sweeps against large enemies. e.g. normally trying to swipe a giant from its feet will result in a sore lower leg on the martial artists side, while using a bo to execute the same maneuver at least has some chance of succeeding and perhaps scoring a large crit; even allowing magical or mithril implements to use that column.