I am so sorry obviously my time in army where tehy did taught Krav Maga and my earlier practise of Tae Kwon-Do have given me simple principle: knife is not defensive weapon. Its last ditch weapon when you use it defensively. As a weapon knife is disadvantage in nearly all scennarios if person sees blade at your hand.
But I am sorry if I don't know any self defense by your standard... Obviously I am ignorant fool when it comes self defense...
Defensive weapons are: Your brains (not get in situations), your senses (its not worth dying over wallet and cellphone), your legs (running away), tazer / mace, that .357 / .45 you carry.
Excuse me to burst your bubble but knife is not defensive weapon, because if you're defensing your life with knife and your not behind assailants back, you've most likely already lost, you're significantly in disadvantage.
That is why I don't categorise knife ever as defensive weapon. But heck if it gives you confidence to carry a knife for "self defense" sure go ahead. By the time you can draw your concealed knife, if you're alive, situation is already over. Besides most assailants come from behind when you have no time to react to get knife, have gun pointed at you or are ganged against you. In those situations a self defense knife doesn't work. I have yet seen a guy who can out run bullet. Do let me know if you see that or can do that.
Knife is primarly a tool and in modern era with fire arms very poor substitute of gun and brains for self defense.