In order for something to be taxed effectively it still has to be controlled by a select group. It would still be illegal for you to grow it yourself or produce it yourself because that would be escaping the taxation process. This would be that instead of the war on drugs, you have a war on people escaping taxation... it would just shift to the battle to a different realm.
Also, government anything is crap, sorry to say. In Canada, they have fields of pot growing in Manitoba for medical users... but as most medical users will tell you, government pot is crap. Most users still turn to illegal dealers to get their medication. I would only be in favor of legalization if it meant I could grow any of these plants myself without penalty or fine, then I could control the strength, quantity and quality myself.