I'm not sure that I'm following the point of this thread very well, but...
You have to be smart about these things. If someone attacks you or someone else with a knife, without warning, and the intent of killing, chances are there's not much you can do. However, the much more likely case is that someone will just threaten you or someone else with a knife. If all they want is your money, throw them your wallet and get the hell out of there. It's not worth risking your life over.
If you absolutely must confront someone with a knife, pick up a weapon yourself.
But really, you must distinguish between advice pertaining to combat, and advice pertaining to self defense. I'm not one who believes that martial arts are actually about self-defense at all; they're about combat. If your question is about how to combat an opponent with a knife, that's a subject that can be delved into. But the discussion gets much more involved if you ask "when should you try to fight someone who has a knife?"
I think you'll find the answer to the latter to be "never" or, at least, "almost never."