Let's say you're about to be attacked by 2 or 3 drunk teenagers.If you're cornered in this situation, maybe you are justified inusing a knife to defend yourself. But, you're all but entirely unableto adjust the severity of your defense with a knife. Slashingclothing or a shallow cut is not going to have the impact necessaryto stop an attacker. You could attempt to cut ligaments and tendonsthat would incapacitate a person, but it would be difficult to dothis without also potentially killing them. With a bluntweapon you can injure to varying degrees, with a knife you cannotreliably do the same.
I know a number of people who have been stabbed or slashed with aknife. Not a single one of them even knew they were being attackedwith a knife. They didn't feel it. I know others who have been hitwith sticks, including myself, and every one of them felt it. Gettinghit with a blunt object hurts! Not only does it hurt, but it canbreak bones. A good shot with a stick to the hand, knee, orshin can incapacitate someone without doing extreme damage.If necessary a blunt object can also be used to cause greater damage.The same is not true of a knife. You can incapacitate someone bykilling them, or nothing. And, even in killing with a knife, it'slikely to take some time. One good crack in the shin with a stick orexpandable baton can drop a person instantly.
In a knife vs. knife encounter, even if you 'win', you'relikely to take serious damage yourself. In a stick vs. knifeencounter, with proper footwork, the stick wielder has a much greaterchance of evading and taking out the knifer with little or no damageto himself.
(The same cannot be said of a machete or sword. Of course amachete vs. a knife is altogether different. However, very few peopleare willing to carry a machete for self defense! An expandable batoncan be carried almost as easily as a knife and can function as a'stick', or long blunt weapon.)