Ok, let's look at the argument and it's opponents - the statement "I want a knife for self defense" usually implies a complete ignorance of combatives and what knife to use. It's obvious that someone coming to a public forum, rather than consult his dojo, sensei, and others in his art, would likely not have a dojo, sensei, or others to consult.
The best advice to that person is don't even try, get training, and use a better weapon - a gun.
And if someone chimes in and says "I can take a knife away and have done so," I'm sure I will take that with a grain of salt. NONETHELESS, LEO and military are both taught disarms to do exactly that. There is no guarantee that two compentent and skill martial artists are involved - so protests that combatants in a dojo practice session get hurt has no practical relevance. Please - what happens in the dojo, stays in the dojo. Not every punk thug on the street is an accomplished belt martial artist. What they are is an accomplished punk thug who ambushes people on the street. I see no reason to embellish their skills when so many wind up dead by the actions of people just like them.
So it's not impossible to disarm and take one down, if you have the skills and the chance. And lots of times, a civilian does that when they pull their CCW or other firearm. About 2 million times a year, if the studies are close - and apparently, there are some here who don't know that.
Big picture, knives suck as a weapon, which is why guns are preferred and have been for the last 150 years, and second, anything else - which the DOJ reports from nationwide police statistics. Even when knives come in third, the majority of violent incidents are domestic disturbances with the wife armed with a kitchen knife.
Making assertions that knives are great as weapons is irresponsible - because it takes years of training and a prerequisite knowledge of barehand combatives..Ownership does not impart competence - and the shooting fraternity constantly beats that drum.
Why the knife using community doesn't is the disconnect - where is it that owning a great knife makes someone a competent fighter with it? That is pure BS - and saying otherwise just feeds the fantasies that could get someone hurt. That is not a responsible position from someone truly in the spirit of bushido.
Learn the best methods of self defense first, then pick the tools to exercise it. Quit getting it bass-ackwards. Buying a NASCAR ready racer don't make you one. Racing on the local dirt track doesn't, either.