If only the careless drivers were those crippled or killed by their poor driving, it'd be a self-resolving issue as you imply, and nobody should care.

However, what actually happens is that careless drivers often cripple or kill innocent bystanders in other vehicles as a result of their poor driving. That's why seatbelt laws and improved vehicle safety features are a good thing.

But surely then you agree they are having an effect. Thus my analogy serves its point - test coverage isn't to be dismissed out of hand.