Cruentus
Grandmaster
EDIT: How then would this be dramatically different than using a gun? A person can claim the gun was used in self-defense since it is evidently not used for carpentry.
Good question.
The difference is that a legally obtained and carried and licensed firearm is acceptable by society and the courts as a self-defense tool; a knife is not.
It is more about perception then anything else. A law abiding citizen with a properly licensed gun is more likely to be seen as such rather then the same citizen that uses a knife instead and claims that the knife was a self-defense tool. There is this perception that guns are licensed by the state for use in a defensive situation (because in a sense they are) where as knives are not.
The thing to remember is that the courtroom is a huge grey area. If one has to use lethal force in self-defense, one has to expect that they will have to defend him/herself in court, and justify those actions to the police, the prosecuter and attorneys, the judge, and possibly a jury. What makes or breaks any self-defense claim is the issue of "reasonableness," or the question if your actions were fitting of a "reasonable" person.
Well, what is "reasonable?" This is very subjective in nature. What is reasonable to you who is a martial artist who understands knives might be completely unreasonable to the people who are deciding your case.
There is this perception that the laws are definative on these issues. They are usually not. The legal "game" is a dangerous game where you are doing what you can to stack the deck in your favor. There is no black and white answer, as it will be the totality of the circumstance mixed with the subjective interpretations of the courts that decides your fate. So, what you are doing is taking any and all measures to ensure that you win in court as well as in self-defense. Because you can count on facing a scary and difficult battle if you do have to defend yourself, you will be glad that you took the proper precautions.