"The Somebody Else's Problem field is much simpler and more effective [than actually hiding something], and what's more can be run for over a hundred years on a single torch battery. This is because it relies on people's natural disposition not to see anything they don't want to, weren't expecting, or can't explain." -- Douglas Adams, on the SEP Field, Life, The Universe, And Everything.
You see, humans don't want to see a rape, nor do they wantto put themselves in harm's way to stop one. Additionally, the more witnesses there are, the greater the chance of each individual figuring someone else will call 911.
If you want to be safe, you need to provide for your safety in a real and substantial way. And not just by carrying a means of self-defense, but by putting yourself in as few dangerous situations as possible, being adequately prepared to defend yourself using whatever tools you carry, and staying aware of your surroundings at all times. I don't know whether the woman in the news story was at a party she thought would be safe, nor do I know whether she had any reason to trust or distrust her attacker. I can, however, be fairly confident in my belief that she could have defended herself. She was apparently able to knock on a nearby door, "yelling for the occupants to call police." This leads me to believe she had a chance to fight back, but was unprepared to do so.
I am not blaming her for this, of course, but stories like this illustrate the incredibly ineffectiveness of the plan of making enough noise to get someone to call the cops. There are certainly folks in the world who would defend you, but they aren't always going to be within earshot. A gunshot will not only stop the threat, but get the police there fairly quickly. A taser or pepper spray may well stop the attacker, as well. Yelling, though, relies on someone forgetting that you might well be somebody else's problem.