No question, you have to understand the law, and also have a sense of realism about how much damage, in any particular situation, is minimally necessary to inflict on an attacker to ensure your safe exit from the situation.
My concern is that someone who flashes a knife at an attacker but lacks the will to use it in this minimally necessary way is in an extremely vulnerable position. Jamming a knife into an attacker's arm, without severing a major vein, or delivering a slash across the face, very painful and extremely bloody but unlikely to be fatal, is probably going to be more than enough to take the attacker out of play... but there has to be that willingness to cross the `blood threshhold', as it was once referred to by an acquaintance of mine.
A cool, even cold, head, is crucial if you're going to be able to do that, and thereby, very possibly, save yourself (with—as you very correctly point out, Ninjamom—the minimal damage possible to the attacker). But pulling out a knife and then making it clear, by a dozen different cues, that you aren't ready to cross the blood threshhold is very likely going to be, well, a red flag for the attacker, whose followup behavior to the visible knife will in all likelihood depend on whether he sees, on the one hand, a pitiless resolve to shove that knife into him if necessary, or, on the other, a panicky realization that the victim doesn't really have a clue what to do with this thing, or the commitment to do it. And if he sees the latter, and realizes that in effect the knife is his for the taking... it's not gonna be good.