You have a point there, since there's no real reason to handcuff and this may turn it off even more. But would be a nice feature just for the realism of things.
As Pilgrim said, if it's done right it can knock out people.
Hence why i said it could be a 50% chance. Or, it can be done only if you aim at the right place, like if you headbutt a guy to his side face it won't work. There are some measures to make it difficult like real life.
As for the waste of ammo, you're right. Just like Rag! said, people tend to not handcuff people because it's not worth it at the moment.
I've thought about this argument throughtout many games...games don't make sense when it comes to this.
If the DayZ player is just a normal civilian, he would missing a lot of his shots. We think shooting guns is really easy because of all the games we play and movies we see. But in reality, there alot of factors to shooting a gun, and i don't think a normal civilian would handle them. Yet, we spray with automatic rifles like AKs and M4s with no problem. Heck even fist fighting is not as easy as it looks. So lets say the DayZ player is a little more than a just a civilian :)