The good folks at Alcoholics Anonymous were so insistent on that that they attempted to talk my mother-in-law out of taking her anti-psychotic drugs she was prescribed to take for bipolar disease. It took a lot of explaining. ("Would you try to talk a diabetic out of taking her insulin? Well, this is the same kind of thing. It's an illness. She's under a doctor's care.") It still didn't sink in with some people because it was a brain chemistry thing. It was cold turkey or no turkey at all with some of those folks.

Now, I doubt that marijuana is any good for treating abuse of another substance, but I also doubt it has been tested exhaustively for that purpose, what with the political and legal considerations and all.