seriously. knives are generally carried around as tools. I collect knives, but simply for the novelty of having nice knives. I carry one around daily with the sole purpose of cutting up fruit.
however, when i walk around late at night in durham, i carry the cuda maxx or vaquero grande. it's saved me from a few muggings; namely, i saw a small balisong or switchblade in the hand of someone sketchy walking towards me (empty street at night). I figure if they have a gun they'd use it first, so they only have their dinky pocket knife. I pull out my 6" folder, tell them its a bad idea and i will wreck their night, and they leave.
That being said, you can conclude that knives serve as a deterrent when it comes to self-defense. Just like a gun. You don't carry a gun expecting to shoot people; you carry it as a deterring force. Same with a knife.
Whenever I get in fights, whatever knife i happen to be carrying at the moment stays in my pocket or sheathed. Under no circumstances should you draw a weapon unless they do so first. You yourself escalating the level of the fight from fisticuffs to deadly force is a bad bad idea.
Then again, say you got jumped 5v1 in a back alley, that's another story.
Of course, this all assumes you're competent with a knife and have had some form of self-defense training. If you're a mall ninja who thinks carrying a knife around makes you badass, leave the blade at home kiddo.