Prostitution should be legalised, and eventually, but not in my lifetime, I think it will be, for the reasons already stated.
The same argument exists for the legalisation of heroin - ability to police it properly, and the erradication of vast amounts of subsidiary crime that go with its use.
Knee-jerkers will argue that it is a slippery slope, but if you think about it, we are already well down the slippery slope in both areas, legalisation is application of braks, before it is too late.