That's what you do when you want to represent the benefit of concentrating your force and momentum onto a smaller area due to the curve of the blade but don't have the ability to represent being better at slicing through the soft bits - when we consider the blades in their original form, at least. Obviously if the system were able to describe the blades' differing facilities for different techniques and targets, we could expect a more accurate representation.
Describing particular maneuvers could be given a bonus dice for effective tactics or effective use against particular armors or enemies - in a sense this is what Called Shots represent.
the smaller area of contact has a multiplicative effect (this isn't unique to katana, but rather a trait of all curved blades assuming the edge is external rather than internal as with a kukri).
I'm more familiar with the shamshir, kopis, and machete than katana as far as maneuvers and particulars of their use, but I know this holds true for those weapons. Of course, I think that a number of straight-edged weapons should have a distinct advantage over curved blades in armor penetration because you don't apply the length of the blade - the longsword was designed to allow a variety of motions, the katana and machete are designed to chop; while a katar, gladius, tantou, cinquedea, or the utilitarian rondel dagger (forefather of the stiletto), which arose specifically to punch through heavily-armored opponents. There are even some martial arts that arose to take advantage of its penetration or a more general development of aiming for the chinks in an opponent's armor.
Remember, not all swords are used for chopping, and the ones that aren't are more likely to be the anti-armor variety.
