Depends on the context. A threat has more impact when the people giving it have a rep for following through. But you're right. Those who are serious are more likely than not to not bother with threats so much. Especially since threatening someone can put them in a prepared position.
that why I never understood the whole 'internet bullying' what would be said, would almost never be said face to face and for it to be 'bullying' it's the bullied that have to make something of it. if the bullied blow it off, it's just idiots blowing hot air.