Agreed - the point is to get someone to let you go or to get loose enough to get away. However you have to hit one of a few extrememly lethal targets to drop a person right then and there. The only other way to get them to let you go is to ause enough pain to make them do so and - as covered above and testified to by someone who has been stabbed - stabbing will not necessarily do that. Nor will a cut with a super sharp knife.
Also, a knife can be sharp enough to do all of the damage needed to human tissues without being super sharp to the point where it can cut with no pain. You don't have to be able to slice through a brick to get the job done.
Again, stabbing vital targets does not mean that the person will let you go right away as it most likly won't have an effect for many minutes - enough time for them to do plenty of damage to you.
So the knife has to be sharp but not too sharp? How do we determine that the knife meets that criterion?
I'm not saying that slashing/cutting type attacks never have a place, I'm saying that I don't feel that they're the most effective method for rapidly ending a threat. You can't rely on pain as a means of "stopping" someone. What happens when either their adrenalized state, or the fact that they're under the influence of a drug affects their pain threshold?
You say stabbing vital targets won't make them let me go or stop attacking? I would submit that if I ram my knife into their groin or kidney, or thrust into the neck and rip through the carotid artery and trachea that I will deter them a heck of a lot quicker than if I just give them a boo-boo on the arm.
Wrong - someone ending up dead and someone not ending up dead is a huge distinction. Also, many states have excessive force laws that will hold you accountable if you use more than the amount of force necessary to get away.
This is also a stong principal in Kenpo - match your attack level to the threat. If it's just a drunk guy looking for trouble you don't need to kill him - just deal with the threat to only the extent necessary. Upper belt levels in many arts have the power to kill someone easily - but it's not necessary in every situation.
No matter what the weapon is, it is not reasonable to say that just because you need to use it you therefore need to kill someone.
Not wrong...
As I said before, deadly force is deadly force. The language of the statutes vary from place to place but deadly force is usually defined as "actions that have a high probability of causing death or serious physical injury." The sticky part is that your actions can be considered deadly force
even if death does not actually occur. In other words, deadly force is determined based on the
likelihood of the method or tool to cause the death or serious physical injury, not on whether the death or serious physical injury is the end result of your actions.
In most places, to be justified in using deadly force for self-defense, you must be able to reasonably articulate that you were in fear of death or serious physical injury. If this is the case and deadly force is justified, it doesn't matter what
type of deadly force you use (knife, gun, empty-hand strikes to vital targets, etc.).
If deadly force is not justified then you cannot use a deadly force tool...period.
To borrow your example of "a drunk guy looking for trouble," if you
did not feel that you were in danger of being killed or seriously injured, you
would not be justified in using deadly force to deal with the situation. That means that if you were to use a knife on him even if it were
just to inflict pain, you would have used deadly force and would be guilty of using excessive force. If however, you
did feel that you were in serious danger--enough to warrant the use of a deadly force tool or methodology--you would be justified in killing him (and no...I'm not saying to perform a "coup de grace" on the guy...

).
I never said that you should kill everyone who bothers you, "matching the attack level to the threat" is generally sound advice. However, that prinicple does not apply when we're dealing with weapons...If you're not justified in killing the person, DON'T USE A WEAPON. Saying that there are situations where it would be permissible to slash someone but not stab them would be like saying there are times when you could shoot someone in the leg but not shoot them in the head...it doesn't work that way.