Marijuana and other psychedelics should absolutely be legalized. (It's really only a matter of time when it comes to pot. Several more states will legalize it next month.) The reasons they have been prohibited have a whole lot more to do with religion and politics than public safety. You will never stop people from being curious about the nature of consciousness and the psychedelic experience, nor should you try, because it can be a truly life-changing and spiritually-beneficial exploration.
I don't know if drugs like heroin and meth (which are basically absolutely addictive and have none of the redeeming qualities of psychedelics) should be legal. But they should be decriminalized. Portugal did this a long time ago and did not see rates of drug use increase. In fact, far fewer people are contracting HIV or dying from overdoses.