As I learned them, the rules of a knife fight are:
#1. Bring a gun.
#2. Bring friends..., with more guns.
I've been training with knives for ~6 months now and I simply cannot envision a time when I will feel confident defending myself with a knife, even against an unarmed attacker with rudimentary empty-hands fighting skills.
That being said, if a knife is the only thing I have to protect my wife/kids, I guess the die will have been cast and I'll do what I need to do. However, I don't have any rosy illusions/delusions as to how it might turn out.