The number 1 reason being the sound that it makes when you pump it. It that don't scare someone, then they deserve getting shot
Problem is, in the dark, the sound of a pistol or shotgun being racked is a perfect way to let the bad guy (Or gal: I must eschew sexist thinking <img src="/forums/images/graemlins/laugh.gif" alt="" />) know where you are. And then, if you don't know where THEY
are you are in trouble.