Try throwing for a little bit and you will see why trying it in a self defense situation is a bad idea.
You would have to be able to accurately judge the distant between you and the BG, this would probably be constantly changing as one or both of you move.
You would have to be able to hit a moving target. Not just changing distance between you, which would constantly change the spin that you would have to use, but the lateral movement of the BG would have to be taken into account.
You would have to carry a knife that had enough weight and edge to penetrate a human body far enough to do damage. Most throwers us wood targets, so their knives stick less then an inch into them when they stick one. Flesh is a easier, but bone isn't. Most targets are approxiamately 18 inches in diameter. The only part of the human body that is that big is the torso, lots of ribs in that torso. If you aim for it you had better hope you get lucky or go for the lower belly. If you want to go for more "Vernerable" area.....well, they are smaller, so now your trying to hit a small moving target.
Knives don't move very fast. Especially heavy ones. If your up against someone with an object in their hand, and this could be ANY object (a coat, purse, plastic restaurant menu, book...etc) they are just going to knock the knife out of the air. Trained martial artists with a little confidence could do it with their hands. Is it going to hurt? Yes, will it be enough to take them out of the fight? NO. Is it going to make them angry enough to want to beat you into a blood stain? YES!
You would have to take all that, and more, into account, in real time, under combat stress.
You would have better luck making a basket from half court, blind folded.