Well, same arguments as with guns, really. Smartly, UK police has figured out that a policeman can't shoot anyone by mistake if they don't have a gun - so UK policemen don't carry them at all. If you reduce a number of people carrying long knives, you reduce the number of people stabbed with long knives. Obviously, just like with guns - people who want to have a knife and use it to commit crime, are still going to.