@ Solarius:
In the context of unarmored targets, I would agree with you. But in XCom, pretty much everyone but civilians are wearing some kind of armor.. Wouldn't pushing a knife through armor require some kind of strength? I would picture a body builder being better at pushing his knife through a resistant material than a little girl.
I agree that applying strength makes more sense for swords and other larger weapons. I would be nice if you could have a "strength modifier" that allows you to add a fraction of the strength to the weapon only. A knife could then be better for a weak soldier (higher base damage, lower modifier), but a sword for a strong one (lower base, higher modifier).
@ivandogovich:
Adding a "facing modifier" to weapons would be a great idea. Something like:
facing modifier: 1.25
Adds 25% damage if you attack on the side
Adds 56% damage (1.25*1.25 = 1.5625) if you attack from the back.
TL;DR: Modifiers for everything!!