I'm very familiar with modern (and antique) knives of all types. I design, make, and repair them.
Knives are not good defensive tools, period. Certainly they can be used for defense. So can an ink pen.
What bothers me about these posts is that many people feel they could actually use a knife effectively for defense. These are usually the same people who espouse that carrying any gun under .40 cal will get you killed.
A knife is a poor choice for defense compared to a .25 automatic.