Carrying a knife is of course risky business. It does not matter if you know how to use one or not. Legally you can face problems if caught with one and if you are caught using one in a fight.
First of all there are rarely knife fights. Sorry, I never seen a "WEST SIDE STORY" scenario. There are usually knife assisted assinations or one sided slashing affairs.
As a user you better make sure you are in the right legally before you pull it out and cut someone. You are best to let everyone know within earshot you do not want any trouble and clearly label yourself as teh defender.
Secondly, if you are ging to use your knife never let your opponent know you have it until he is cut. The severity of the cut is determined by the situation.
In a combat scenario unless you know how to use a knife you can cut someone and their rage will not let them feel the initial cut if the cut was not effective. And if they pull a gun in response you better be quick and really efficient.
There were some pretty good comments on this post but some guys on other forums I have seen are caught in a martial arts fantasy world.
Knife training is as essential in today's society as it was in Fuedal Japan, Ancient China, Indonesia, Okinowa, Midevil Europe, Africa and Colonial America.
As a martial artist in today's ultra violent world yet so-called civilized world you have guys who would not hesitate to carve you up or shoot you in a second. IF you are not training with a knife, if you do not know how a real knife attack will feel or how a knifer moves you are not practicing martial ways you are indeeding practicing "ARTS". As one of my fellow senseis states "you want art -get a brush"
Some people are so wrapped up in the non-weapon carry martial artist bag that they dismiss knife carrying as being beneath being a real martial artist. I think they are sadly mistaken.
As the kempo instructor ably stated he carries a knife and a gun. His mentality is of a man who studies a martial way if I may be so bold. I believe in the option of weapons first and hands second if the situations calls for it. It is better to have a knife (or gun) and not need it then to need it and not have it.