I'd have a hard time quantifying the number of police reports I've read in my career, but I'd be surprised if it wasn't over a thousand. I can say that I've never read a police report where a sex crime was solved because the defendant was required to register as a sex offender. For example, I've never read a police report where a stranger-to-stranger rape occurred and the defendant was found because the police went to all the homes of the sex offenders in a six block radius - or showed the victim the photos of all the nearby sex offenders - and "Bingo! That's him!"
Obviously, that's just one felony attorney's experience, but I would note that I habitually read the local news, and I've never seen the story where a sex offender registry helped solve the case. Maybe they left it out? I can't imagine that the police spokesperson wouldn't trumpet that fact if given the chance to. Has it happened ever? I guess I'd say it must have happened somewhere, sometime, but we don't generally justify millions of dollars by saying it's helped solve a crime once a decade. If you give me a couple of million, I can solve a lot more crimes than that.
Okay, so they may not solve crimes, but do sex offender registries reduce crime? You don't have to rely on my experience for the answer to that one. Studies have been done. A discussion of those studies can be found here.
Sex offender registration does create crime, though. Not just in the sense that it might drive sex offenders into homelessness, and homelessness can lead to more criminal activity. Rather, it creates a crime whenever a convicted sex offender doesn't register. Then there's the cost of housing them in jail, appointing a lawyer, the time and cost of police, judges, jail deputies, courthouse staff, probation officers, and so on. People who favor being tough on crime, I wonder if they ever think how the government is picking their pocket - via taxes - in order to prosecute victimless crimeswhen doing so doesn't help reduce - and may actually increase - crimes against real victims. Or the lifetime cost of making it hard for someone to find a job, because at 20, he had sex with a 17 year old, who was just over three years younger. We do so much damage to people and for so little purpose. And we do so because it's the law, as though the law were a reason for doing something, and not just a tool to get to the just result, and because no one with the power to do such damage ever questions it.
Post-script: High rates of sex offender recidivism are a myth. I wonder how many prosecutors know that. You'd think it would be an important consideration in making plea offers. Details here.