These days I carry a knife as a last ditch, everything else went to heck, resort. Back when I was 10' tall and bullet proof my Spyderco Mariner was my only defensive tool, and it was a poor choice for that role but...
While riding cross country on annual leave, I happened into a biker bar that I did not know was notorious for brawling and a truly wild crowd. Everything was quite inside but when I finished my beer and left, I walked into a melee in the parking lot. There were at least 20 leather clad bikers going at it with fervor. I turned to go back in the bar when someone grabbed me by the shoulder, spun me around and said "Where the #*&% do you think you're going!"
I have never deployed a knife so fast before or since. I had it in the outside pocket of my right Nasty Feet boot. The foot came up, the blade went "snick" and I said "Back inside to finish my beer friend." I got lucky, he let me go and the cops showed up to break up the fight. Had I not had that knife I'd have probably taken a beating and spent the night in the pokey.
I guess that counts as defense with a knife.