Here’s an article from abcnews outlining reasons for legalizing the use of drugs in the US. Their main points revolve around the costs of maintaining a drug war and all the dirty money that drugs bring in. i.e. legalization means cheaper prices, less crime, less corruption; drug users are generally harmless. Take someone who just smoked a bowl and someone who’s completely trashed, hand them a baseball bat and tell them to have fun. The drunk person is likely to start smashing things, while the high one will probably just look at the bat and start laughing.
Drugs and prostitution, you could make much the same arguments for the legalization of both. My views are that both should be legal and talked about. People should be informed about the consequences of their actions and have the right to make their own decisions (after reaching adulthood of course). People will always be able to find drugs or prostitues (the world’s oldest profession), so we may as well take the money away from the criminals and put it back into society (via the government).