If all you've got is your knife, why the hell would you throw it?
Against humans, throwing your knife can serve as a distraction and hopefully cut someone, but as a knife-throwing hobbyist (hangs head in shame) I gotta say it's not a good idea. Plus you've got to be ready for the possibility that you'll now have to incapacitate your opponent barehanded, or run and retrieve your knife -- That is, IF he doesn't get it first.
Against zombies, the idea is even worse. Don't carry around your bracer of throwing knives unless it's just to preform sticks out of boredom.
If you drop a zombie, does that count as a kill?
...Think about it.