In my experience the sound of a pump racking a round into the chamber is an attention getter. Thus far all the thugs that I've drawn down on with my 870 have gotten very peaceful upon hearing the distinctive "slack-slack". I don't know if the next suspect will be brought into compliance so easily, each situation is unique. Having been on the receiving end of a suspect chambering a round on me (at 0430 in the morning) it got my attention but didn't cause either me or my partner to faint. You can put me down as one who generally believes in the intimidation value but backs that up with the willingness to use the 00 buck that is now in the chamber.
All that is neccessary for evil to triumph is for good men to do nothing.