What Are The Arguments In Favour Of Legalising Or At Least Decriminalising Drugs?

There is no point in criminalising addicts.  This does not aid them in getting help to quit their habits any sooner.  

The drug market reacts to the criminal status by keeping prices high.  This makes drugs more expensive, and motivates addicts to commit crimes to feed their habits.

The high prices makes dealing drugs very profitable, particularly for the people at the top.  Drug makers are motivated to come up with more and more addictive substances.  They are also motivated to commit acts of violence in turf wars.

Vast sums of money and government effort are put towards putting dealers and users in prison.  This could be better spent helping addicts get clean and educating people not do start using drugs in the first place.

Making drugs legal (it's argued) will not send out the message that it's okay to just go ahead and use them.  Prostitution is legal in your home, but society still frowns on that. Ditto for teenagers having babies or people living rough on the streets.  All legal, but culturally unapproved of.