Maybe after a certain point in agi, say 21 or 24 you start to do more damage to enemies from behind or something? To promote the agility footwork. Wouldn't apply to bows. This dmg buff could apply more to cut and pierce, as it seems it may be too easy with blunt, but it could still apply to a lesser extend. Also swords and daggers are more assassin-like so this would make sense.
Or maybe something like a cut or pierce attack to the back staggers an enemy for longer...
Just seems strange when you sneak up on a light-to-medium armored foe, and impale him through the back of the neck with a sword, only to have him do a random sideswing while turning around and take you out one hit...
Or maybe a slight increase to movement speed when running backwards! I dunno, there are many things we could come up with to buff them that would be much more creative than just adding dmg to agi builds or whatever...