In my view, the main reason for making prostitution legal is to allow victims of trafficking and sex crimes to seek justice against their abusers without themselves being prosecuted. This doesn't mean that prostitution isn't still a negative and that we shouldn't try to eliminate it where possible.
I guess to be specific I think the police trying to expand anti-pimping laws to classify anyone who does business with a prostitute as a pimp are clearly detrimental to the well being of sex workers, but I don't think laws which discourage men from buying sex to begin with are necessarily more harmful than helpful.