This is true, because , as you stated, some states have already decriminalized it. IMO, prohibition on MJ doesn't work , in the same way that it did not work for alcohol.
I believe the prohibition on marijuana is working -- keep in mind that marijuana has been illegal now for what, almost 70 years? And I don't see and end to it, do you?
Alcohol prohibition, on the other hand, lasted a mere 13 years before it was overturned.
One other thing. People are calling for the legalization of one drug, marijuana, and expecting big results. Imagine if, during Prohibition, only wine was made legal. Would you expect big results by legalizing just wine?