There are two meta arguments for legalization. The first is based on Natural Law or Natural Rights: people have a right to manage their own bodies as they see fit, so long as they do not interfere with the rights of others to do the same.
The second is that a cure that leaves the patient in worse shape than he or she was before the cure was applied makes no sense. Yet that is exactly what the War on Marijuana Users does. No matter what marijuana does, no matter what you think of it, the damage left by the Drug War as it has blown through society — hundreds of thousands of families broken up every year, tens of billions of dollars a year expropriated from American taxpayers, the creation of a huge and totally unregulated distribution system, and the general and systematic erosion of our rights, has been way out of proportion to any possible benefit.