Cruentus
Grandmaster
I was wearing my heavy leather jacket one day while working with a knife -- it slipped and I stabbed myself in the forearm hard enough to cause a bruise, but it didn't penetrate the leather. (But it was a heavy leather jacket).
I have a friend who wears motorcycle jackets (leather) with the heavy-duty zip-up sleeves - the metal zippers add a bit of protection to the inside of the forearm -- a prime target to experienced knife-fighters.
One thing about leather; although I said that earlier that clothing and leather can be helpful, I want to reiterate its usefulness. Leather helped me out once in an actual knife situation where I took the knife out of someones hand; but I was wearing leather gloves and that made all the difference between me being cut and not.
If Motorcyclists wear sturdy leather to guard them in case they wipe out, then logic dictates that it might be useful against sharp weapons.
No garuantee's mind you, just helps your probability a little is all...