While I fully agree that skill and training are always critically important in anything you ever do, most certainly including self-defense, I find the rest of your argument somewhat bizarre.
Producing a knife introduces a weapon, yes, but a weapon that you're holding. Disarming a knife isn't exactly trivial, even if this 'determined sociopath' is well trained in unarmed combat. This means that the attacker has a problem - his victim has a potentially lethal weapon in hand, he does not, at least not yet.
You are perfectly correct that the assailant might produce a weapon of his own in this situation, perhaps a firearm. But come on, is there any reason why he could not do that even if the victim did not have or produce his knife? No, there absolutely is not. And that leaves us with the situation being one of two options: 1) victim has a knife, attacker may have any or no weapons or 2) victim has no weapons, attacker may have any or no weapons. Which of these two scenarios sounds preferable to you guys? I'd go with number 1 any day myself, training or no training...
With that said, I wouldn't want any "pocket knife" for self-defense. Even the best one-handed open folders are unreliable and slow compared to a good fixed blade, in a stressful, surprising self-defense situation where complicated actions can be extremely difficult to perform with reliability and effectiveness.
Like maximus said, it's your skill and training that provide protection against an assailant. The weapons you may carry are only tools, but not all tools are equally effective.
