I think they are beneficial. Firstly usually attackers stalk their prey for at least a few minutes, hours, days etc. A woman who has taken the time to learn some techniques may well exude more confidence when out walking alone, generally attackers will go for meeker looking victims, those they think will not fight back.
Secondly most attackers want their attack to be quick and as quiet as possible. Even if the woman does still get hurt she is more likely to get away if she puts up a fight and makes noise.
A lot of self defense classes are not about being able to put a guy down, it's about giving vital time to make a run for it or to hail help from somewhere else and giving a woman the confidence to do that.
I was a publican for a long time and I was taught some basics from someone who frankly you really wouldn't want to meet down a dark alley if he had a grudge against you. He taught me ways to just simply get away from an attacker and more importantly how to avoid finding myself in that situation in the first place.
Yah your right on in this post.
Criminals know how to read a person so they can pick on the easiest target.
a lot people don't realize it but things like the way you walk and your posture, the look on your face, are all thing a lot of criminals consider when picking out a "target".