I actually took a couple knifefighting classes from one of Janich's instructors before... They teach you to deflect the attacker's knife and cut critical tendons. Since you're cutting to immobilize and not to kill, I'm sure the courts would look favorably on someone defending themselves in that matter. That being said, I'm pretty sure I'd forget everything I learned in a high stress situation lol. I just keep a machete in my car and I figure if a bad guy sees a big blade, I won't have to use it. Actually that machete is another example of an odd knife use; I mostly use it to make parking spaces for myself when there are overhanging trees or overgrown bushes.
Your questions does make me think, though, if I had to use something aggressive looking like my Cold Steel Black Talon 2 for defense, I might have a hard time convincing people that I really carry it because it's one of the best cardboard cutters in my collection...