The point is they are never going to just fall over. If you take enough trauma, then you could pass out. That doesn't happen too much with knife wounds.
I think this is very true if you look at statistics on modern knife wounds, but misleading in terms of medeival warfare.
First, in support of Zachs statements, I have seen this with my own eyes. Growing up in the French Quarter in New Orleans I've seen dozens of fights where one or more of the combattants had knives, and I'd say in 9 of 10 cases the knife had almost no effect on the fight until afterword. Usually people dont even notice they have been cut or stabbed, even if they have been seriously hurt. That makes the pocket knife a pretty crappy self defense weapon, it seems fairly unlikely to STOP someone from attacking you, other than by intimidation, but it's quite likely to cause serious injuries which could have very bad long term consequenses. Almost the opposite of a cattle prod, say.
This happened to a friend of mine six months ago. He was drinking (and pretty drunk) at a bar on the balcony overlooking decatur street near Jackson Square in the French quarter, when a guy called him out to go fight outside. They were walking down some stairs to the street when the guy jostled him. When they got downstairs, he beat the guy into unconsciousness, then walked back upstairs. Only then did his girlfriend notice that he had blood on his shirt. It turned out his right lung was punctured in three places from stab wounds, (apparently inflicted in the stairwell) and the guy he fought is now in jail for attempted murder.
And yes this sort of thing is borne out by FBI statistics. However, it's worth considering that you get a lot of the same kind of thing (running for blocks or fighting for several minutes) from people who have been shot with small caliber firearms.
And on the other hand, we are really talking about modern knives. Pocket knives are designed primarily to look cool, to cut ropes and things, and to fit in your pocket. They have short usually single edged blades and are weak and will often snap when they meet strong resistence (ribs)
This is not the same with medieval daggers.
One year on Mardi Gras several friends and I witnessed a bouncer at a local pub, a big man, get stabbed by another guy with a bayonett. He took two steps and fell down dead.
I know it was a bayonett, with an 8" blade, because I almost had to testify at the manslaughter trial. The guy who did it ended up pleading guilty and is now in Anglola.
'Point' is though, when it comes to dagger fighting, it's worth keeping in mind that not all knives are created equal. If you look at ancient daggers, these are long, stiff, weapons which WILL reach internal organs if you thrust hard. This is especially true for the more multi-purpose daggers (as opposed to specialized armor piercing weapons like the Rondel) Look at the old Roman Pugio for example. That thing is like a garden shovel. It's extremely wide yet curved elegantly away from a very sharp point, and of course sharp on both edges. This thing is going to split open organs like apples and cut arteries like rubber bands going in to almost any part of the body.
I think you cannot separate any kind of knife fighting from anatomy, (and you better be aware of your intentions if you are using a knife) but there is a huge difference between a buck knife with 3/4" wide, 3" long blade, and a double edged ballock dagger with a 1 1/2" wide, 10" long blade.
Thats why IMO modern knife fighting emphasizes cuts to the arms and such, while the Fechtbuchs go for the killing blow.
Jeanry