I do believe that it sometimes works, but it's effectively like missile defense: Immensely more expensive for the defender than for the attacker. If the attack

I do believe that it sometimes works, but it's effectively like missile defense: Immensely more expensive for the defender than for the attacker. If the attacker has little to lose (e.g. because they're anonymous, doing this massively against many unsuspecting users etc.), the chance of them eventually succeeding is almost certain.