Mises.org has a great write-up on drug legalization. Vedran Vuk does a great job spelling out the current legal and nonlegal costs of doing drugs. He demonstrates that the legal costs of doing drugs is significantly, if not overwhelmingly, lower than the nonlegal costs of doing drugs. The illegalization of drugs introduces violence to the drug scene, something unneccessary and costly to our society.
Bottom line: Rehab and responsibilities are far better for society than prison and the police state.