I have not used a knife in a defensive scenario, nor would I ever unless it was the absolute last option available. Anything can be used as a weapon, and if battle is at close quarters with no firearm available, I'd prefer a bludgeoning weapon or a good quality pepper spray to create distance and time in my favor. I have, in several incidents, used bludgeoning weapons with some effect. I have, in many instances, used pepper sprays with devastating effectiveness. I have only personally seen, after over 250 documented "sprayings", one complete failure of the pepper.
Most states allow the average citizen to carry pepper for defensive purposes, so let's not forget that option.
To be legally justified in using force, you must show a reasonable escalation of force options and/or that you entered the scenario at an appropriate level of force. Pepper sprays, being the lowest level of force beyond soft hand techniques, are easy to justify. Additionally, they will, in MOST instances, create distance, from which you may find an avenue of safe retreat.
Beware, however! Many over-the-counter aerosol subject restraints are weakened "civilianized" versions of police/military formulations. Do some research and get a good quality product, just as you do with your knives. When/if you acquire an aerosol, acquire at least two cans. One should be used for training, as they do require some practice firing. Lastly, get some professional training/advise on the proper use of an aerosol subject restraint - contrary to popular belief, it is not like spraying a can of hair spray.