As a student of knife combatives for a long time, I must say this:
There is NO such thing as knife-fighting. Because in a TRUE knife fight, the loser is the dead one and the winner is one that dies minutes or hours later.
There is knife combat, knife defense, and defense against a knife-attacker, but no such thing as knife fighting. Most violent incidents involving knives that I observe are either outright killings or attempted killings, OR someone using a knife as a defense against another assailant who is armed or unarmed.
Apart from sanctioned ring events, knife fighting is simply too dangerous in the real world. Even a person who has no experience with knife techniques can seriously ruin somebody's day, permanently.
A stab/slash is just like a slap/punch. During a regular fistfight, even if you don't really win or put a hurt on your opponent, how many punches do you normally land on the opponent, at contact range?
A LOT. Even if he has you in a bear hug and your punches don't carry much energy as usual, but still your fists make a lot of contact with soft parts of his body.
Now envision this same fist but clutching a razor sharp metal instrument. Suddenly, what only appears to be weak punches becomes one fatal blow after another as steel pierces flesh with ease.