Once I heard an example that really made me think, countries where Marijuana is legal there is a lower rate of crime and violence, when compared with countries where it is illegal. (Example: Netherland vs Brazil)
I know it might not have a direct correlation, but it definitely is interesting.
My opinion is that it should be legal for many reasons, but especially because the society already has rules to prevent the abuse... (like alcohol, don't drive if you drink).