What do you suppose criminalizing marijuana use does to encourage marijuana use? Would consumption go up or down if it were legalized? Has it? What is the observed effect on hard drug use?
Let us assume that MJ were legalized and treated like alcohol is now. It would be made into cigarettes in legal factories with quality control systems so there would be consistent dosage and no unknown additives. It will probably be sold side by side with alcohol in what are now called liquor stores.
Kids would have a hard time getting any, though they might get some. They get some beer now, often with the assistance of their parents.
The government would stop the silly lies about its effects and the people who need it would have no problem getting their medicine. The black market would lose its biggest money maker and MJ smokers would have no need to do business with those guys.
Many lawyers in the prosecution and defense business would be out of work. So would many police, prison guards, testing companies, drug counselors and drug dealers.
Tens of thousands of people would become productive, tax paying citizens again when they are released from jail.
All in all, sounds like a really good idea, whether consumption goes up or down. My guess is that there will be little change, but we will never know for sure since we have no accurate numbers on use now, since it is all black market.