I notice in reading the varied posts that there are clearly two lines of thought on this. Most posters are thinking along the lines of self defense in their homes, while the police officers, both active and retired, are thinking along the lines of "in the line of duty".
These are clearly two entirely different circumstances, and require different lines of thought and preparedness. The situations require different mind sets, one is clearly defensive and the other is offensive.
In my case, the many, many times that I racked a shotgun and pointed it at someone, ready to pull the trigger in an instant, it was clearly offensive. They were doing something against the law and it was my sworn duty to stop them, and I was hunting them. By hunting them, I don't mean that literally in the sense that I was out to kill them. I was after them to take them into custody so as to protect life and property and to stop them from their illegal deeds, whatever that may have been at the time. I've arrested murderers, rapists, child molesters, burglars, etc. at the point of an 870, and never did I expect the sound of the shotgun to do the job. It was up to me to do the job, and the shotgun was just one of my tools. Looking down the barrel of an 18" shotgun, after hearing it racked, or not, is very intimidating, unless the person is so intoxicated, or so under the influence of drugs, that they have no comprehension of what is taking place.
Was I prepared to pull the trigger each time I pointed my shotgun at some perpetrator? The answer is "yes", each and every time. Did I want to? The answer to that is, "no", but I would have, if required.
This isn't a subject for speculation, as Fast Frank clearly posted in the original question. Unless you've been in the situation, you can't guess as to what will happen, as people react differently. Even the same person won't necessarily react the same on two different occasions, in the same exact circumstances. One should never expect the sound of the shotgun racking alone to scare someone away. Sometimes it does, but sometimes it doesn't, and it only takes one miscalculation to result in tragedy. If you aren't willing to pull the trigger when you're confronted with a life threatening situation, then the gun shouldn't even be involved, whether it's a handgun, rifle or shotgun.
Hope this helps.
Fred