According to my instructor, Barry Cuda, Paul Vunak used to have his students practice with colored magic markers in place of knives. Afterwards, you would look at your limbs and body to see how many times you'd gotten "cut". The point being that anytime a knife is involved, expect to get cut.
Going empty hand vs. a knife should be the absolute LAST resort, meaning your life depends on it. If you can escape, then escape. But if you have to defend yourself then you should have the proper training under your belt.
What frustrates me about the way knife defenses are taught in some systems is that the student is never taught how to use a knife. If you don't know what a knife can do and every angle that it can come at you, you can't be expected to effectively defend against it.
The classic overhead stab or reverse-punch-style thrust is not realistic because the chances of someone coming at you like that is slim at best. The knife, like the fighter, is an alive thing. Unless you train with an alive mindset, you will be stuck in your fixed patterns, unfit for real world combat.
Naturally, the chances of you facing off against a fighter trained in FMA or some other knife fighting system are also slim. But if someone plans to stab you, they're not going to make it easy for you to defend. And they're certainly not going to telegraph their movements by first getting into a karate stance before they thrust at your gut.
Vic
www.combatartsusa.com