It's an hysterical (oh the irony of that word) response to the idea that rape happens unexpectedly to women in dark corners at the hands of complete strangers. Sadly the majority of rapes are carried out by people already known to the victim.
The only decent criticism I've seen of them is what is the rapist going to do when he finds out he's been... barbed. It may well make it worse for the woman, especially in a gang rape situation. And if they get to be common, rapists will start checking for them and removing them.