If people want drugs - they will get them, illegal or not.
If they are legal you lose the crime and violence and gain tax revenue... if they are illegal people get them anyway, risk getting contaminated and dangerous substance, kill each other over them, and there is no tax revenue and, in fact, a tremendous expense to enforce the drug laws and catch and keep all these people in jail.
Sure, I can see incredibly dangerous drugs being illegal - as with dangerous prescription drugs being pulled from market. But I'm not aware of very many, or any, marijuana OD related deaths.
*shrugs*
But, if you think some guy who smokes pot once a week deserves to go to jail and cost tax payers thousands of dollars to rot in there, then comes out unable to get a job due to a criminal record and resorts to a life of more crime... more power to you.