With previous laws, police could use threat of prosecution to obtain testimonies. Now they can't. With previous laws it was not a crime to be a prostitute. You could go to the police and make accusations against a pimp without incriminating yourself. Police could only arrest you while you were in the act of working on the street or incall. I think it's not always the fear of arrest, but the police mentality and the impression that they won't be treated like real citizens that prevent them from going to the police....unlike before it does provide the option for sex workers to come forward against their pimps or bad dates/violent clients as it does not criminalize sex workers anymore.
Even if they are not criminalized, workers do not trust the police. In Sweden to get any help, a prostitute has to quit the job. If people learn that you're a prostitute, your life is pretty much over unless you make a show of quitting. You will be evicted, lose your kids, be expelled from college and lose any other regular job you may have. If they are illegal immigrants they are automatically deported.