What I found interesting in doing some reading, and its a bit off topic, is that random crime is not so common. So while the discussed cases are interesting to think about there are many others that might be more likely scenarios esp. domestic violence and altercations between those that know each other. In these situations it might be difficult to distinguish victim and perpetrator from each other and both might be likely to turn on someone that intervenes.
There you go, Dan, talking common sense...sheesh!
Seriously...everybody's favorite "what if" self-defense scenarios involve random crime, not the more likely situations where, as you say, victim and perpetrator know each other. My guess as to why is that, while the former situation is more rare, the latter situation is more frightening -- because if you accept the truth that most violence is not perpetrated by strangers, you suddenly start to feel a lot less secure in your snug little world. People would rather train for "self-defense" against a nonexistent threat than accept that they may be living with a real one.