The question for voters is not whether marijuana is good. It is whether prohibition is good. It is whether the people who use marijuana shall be subject to arrest, and whether the people who supply them shall be sent to prison. The question is whether the war on marijuana is worth what it costs. Initiative 502 says no.
If marijuana killed people, or if smoking it made people commit violence and mayhem, prohibition might be worth all its bad effects. But marijuana does not kill people; there is no lethal dose. Marijuana befuddles the mind and stimulates the appetite, but it does not make people commit arson and brigandage.
Some people abuse it, just as with alcohol, but cannabis is less of a social problem than liquor, wine and beer. And society manages those as legal, commercial products.