Earlier quoted context omitted.
This is the rationalisation of a legal kludge that is only necessary to assuage consciences that the state has not become "immoral". The facts on the ground are that, dissuasion or not, these economies will never go away ("oldest profession", after all). Full legalisation would allow for complete oversight of such murky sectors, which would ensure everyone's safety better than the current arrangements, at all levels.…
I agree that prostitution should be legalised and regulated to make it safer for everybody involved (because I too don't think it'll ever go away). Cannabis? Probably — I don't know that it's that much worse than tobacco. Full-on legalisation is not universally desirable, though — there's always heroine, cocaine, crack, meth... — and the asymmetric legality thing is still a very useful tool in those circumstances.
There are some truly horrible substances, but with current approach heroin and weed are sold by the same guy.