The way I see it is that it's an extra tool in the arsenal. If rapists start to realize all their potential victims have this device, they'll have to resort to other means of releasing their energy. Rape is actually a bad enough situation and it'll get escalated regardless, so using any tool at your disposal including this device is better than having nothing to stop a rapist. If they murder you, well that's just one more criminal charge on the list.
Of course, nothing is stopping a rapist from checking before doing it, but rapists don't really think rationally when all the blood flow is going down there. They are more intent on the act of power rather then checking the boxes to see if everything is in place.