The katana is actually a very bad sword for hitting anything hard; once the brittle edge breaks, the soft spine of the blade bends in on itself. Pure steel swords were far more effective because their blades were of a more uniform rigidity, making sure that the impact was spread over the whole sword. Katanas were sharper than longswords, but they were also shorter, heavier and more delicate.
http://www.youtube.com/watch?v=5Hy_A9vjp_s#t=5m55s
I guess they both have situations where one is better suited to the job than the other. Katanas obviously weren't meant to parry medieval European swords.

Leather is for binding individual sections of Armor together, and providing a lining between the wearer and the metal work.
Like the padding in plate mail, to give the user improved comfort wearing the armor?