I've never liked this sort of argument. For certain things, drugs being a prime example, people say that because something still goes on it proves the law/legislation doesn't work.
Well murder still happens, does that mean there's no point having a law that punishes murderers?
Rape still happens, should we stop punishing rapists?
Yes, street prostitution still exists, but I believe at a much lower rate than it did before legislation. Now there are other reasons for that (advent of the internet and adultwork for example), but I believe legislation against it and improved methods of detection have played a large role.
That wasn't really my point... I was more pointing out that they don't really enforce the laws they do have with much vigor. For example, if they really wanted to get rid of RLDs totally, they could put constant heavey police patrols on all of them. Of course, they don't, because that would cost a fortune and they have other things to do with their resources.
To answer what you've written though, I think that's an overly simplistic response. Any law is an instrument to try to create the society that those in power want to see. Like any intervention, it will have side effects and will be more or less effective in achieving its intended goal.
To take your example of the law around crimes of violence. I don't think you'd find much dissent that a society where people aren't at liberty to attack one other is a good thing. There also aren't many side-effects it outlawing murder. Outlawing it doesn't create a black market, or encourage more killing.
Drugs, on the other hand, are a very different matter. Most people who take illegal drugs are willing participants. Banning them creates a black market, encourages criminality and increases harm. The law isn't particularly effective in stopping their supply, so the argument that outlawing them is effective in protecting people is a weak one. It's become increasingly clear that those who argue for continued prohibition do so on moral grounds, dressed up in the clothes of harm reduction.
Those who are arguing for a ban on prostitution are doing the same. They want to live in a society where it's illegal, and they're making all sorts of 'protecting the helpless' excuses to cover that fact up. That's why rational counter argument is ineffective. They're like religious zealots. They see in black and white, and can't entertain the idea that somebody may voluntarily sell sex for money.
