Defence. Why would you be carrying a knife in the first place? Although I do understand what you are asking, because if you do come accross in a situation where someone attacks you with a knife then you do manage to get it off them then I suppose you do need to know where to attack someone with it so that you dont really harm them as you will get in trouble with the law, but to immobilise them. If you only knew knife deffence then you could get the knife off the attacker then you are in a sparing match where he could win and get the knife back anyways. So both are important in their own respect.
My club is running a course in a few weeks time on stick and knife defence and how to deal with them. So that should be interesting and fun.