You Don't Use Your Emergency Flashing Lights To Warn A Tailgater.

The other day I saw a driver put on his emergency lights to advise the driver behind him that he was following too closely.  Tailgating.
It seems that this procedure was taught in the United States by driving instructors back in the 60's.  Why has this technique been dropped?
Isn't it an emergency when one tailgates you and therefore puts you in danger of being involved in an accident?  We use our emergency flashers for many situations, such as when you see an obstruction ahead and will make an emergency stop.  When you merely use your brake lights, usually the driver in back of you feels that you're merely slowing down and will then resume your speed.  But if you use your emergency flashers the driver tailgating will know that there's an emergency ahead and will slow down and back off.  (He doesn't know that he's the emergency but you know you're in danger).  Once he realizes that you didn't slow down he may catch up and start tailgating again.  If you repeat the procedure he should eventually get the message that you think he's tailgating and putting you in danger.
This should become a universal signal to tell the driver in back that he's tailgating -- among other reasons. If you can't see all of the front tires of the car in back of you -- then he's tailgating.