San Francisco will be voting on Proposition K, which will decriminalize prostitution, which is different than making it legal. And this really isn't a discussion on Propositon K, but more on prostitution. I have spent enough of my younger life in bars to know that if you want to get laid, just strike up a chat with the guy/chick next to you. Then the sex is free. No money exchanged, no laws broken, no need to legalize anything.
My guess though, (and it is only a guess since I have never hired a prostitute, male or otherwise,) is that the benefit of paying for sex, is the guarantee you get what you want the way you want it. If you pick up a one night stand, there are no guarantees because the other person may be more interested in their own big O instead of making sure your needs are met. You might be expected to give instead of receive. Probably not the case if you are the sex for pay consumer.