We had an incident a few months back that reinforced the idea of atemi. The person acting as uke had said a few minutes earlier "I doubt if atemi would do very much", when both uke and nage mis-timed their technique. Nage entered too deeply with an palm-heel to ukes forehead, nothing too hard, but there was a slap and uke fell to the ground at his feet.
I went straight across to see what damage had been done as I'd only seen the contact out of the corner of my eye. Nage was apologising and uke said rather sheepishly "I guess it really does work; after that hit me you could have dragged me wherever you wanted"
It wasn't a hard hit or malicious, but it certainly proved how effective even a simple atemi can be. I don't think either uke or nage in this instance will quickly forget the reaction.