I don't carry a knife for protection because it's hard to justify the kind of a mess you are going to make with a knife in court. One reason for this is the closer your "carry knife" is to being a combat weapon, the more likely you are to be perceived by the prosecutor, their "experts" and the jurors as an aggressor. The less like a combat weapon it is, the more useless it is as a defensive weapon. I know it sounds crazy, but in the United States, it is actually easier to prove Self Defense (which is a legal term with specific definitions) with a gun, especially if said gun ran the intruder/attacker off without a shot being fired, which is what happens the majority of the time.
