The purpose is not to show what is statistically likely but what is entirely possible.
I think many people do not have a realistic understanding of gun shot wounding mechanisms. Too often I hear people say that a .45 hits like Thors hammer and nobody can keep walking after taking one, and "all you need is one bullet." But this I think fosters a dangerous mentality where one may fire off a round and then hesitate to continue because in their mind they know that round should be Thors hammer yet the bad guy might not even seem to react to being hit.
In that video from Dr. Grabinsky, he shows a video too where a robber shoots an unarmed victim, and the victim does not even fall on the floor. He runs away, come back, sits down, stands up and walks around. So even to a regular guy who wasn't any sort of commando, he did not feel like he needed to keel over just from taking one bullet.
And I actually doubt police usually need many shots either. Are they more likely to encounter this situation? Of course they are. They go on the offensive and encircle known dangerous people as a matter of their job whereas most of us try to avoid such people... and these people know they are only getting out in a body bag or in handcuffs and some, (I suspect the minority), will choose the former over the latter.
It is not to say that you will need 100 rounds most of the time, but one should know not to expect when a bullet hits a target to necessarily see a response at all.
It depends too on the motivation of the attacker. The attacker who attacks selfishly to take what you have for himself may be inclined to give up when shot, but that guy who is delusional and thinks you stole his job and his girlfriend and ruined his whole life may not want to stop until you are dead. Even if you never did anything to this guy, some people are totally delusional.
Historically, most mass shooters have shown themselves to be wusses who commit suicide at the first sign of resistance, but these guys have mainly been homegrown people on antidepressants.
But looking at the terrorist attacks in Mumbai, India and in the cartoon shop in France, I anticipate it is only a matter of time before it happens here also and in a place other than a military base. And these guys are not our local wusses that start mass shootings. They are there because they seethe with hatred and these guys will probably shrug off even fatal hits and keep fighting because they know they are going to heaven and believe they are "fighting the good fight."
Of course, a regular Joe has no legal obligation to fight these people and could run off if they had the opportunity. But even if ones inclination is to run off, you might not even be lucky enough to have that opportunity if you are trapped on their killing floor. And if regular Joe does bare witness and fail to act, he should be aware that society will judge him harshly. Just look at what happened to the business owner who had a gun across the street from the I-Hop shooting in Carson City. He suffered some castigation for his inaction (at least on the internet) and not long afterwards he closed down his BBQ joint. In any case he definitely seems to have wished he acted differently based on his own statements. Was the castigation fair? No, I don't think so. If he intervened there was a good chance he would have just been another dead guy. But the castigation still happened. We all live with the consequences of our actions, and the consequences of our inactions.
You're right of course, that the majority of us will probably be okay with a 5 shot revolver and most attackers are not interested in fighting to the death. I'm just saying that I think it is possible to end up in emergency situations where you might like to have some more rounds. Even I carry a 5 shot j-frame often, but I think carrying more ammo just in case is an entirely reasonable choice based on the entirely reasonable possibility that 5 will not be enough.