On that, you and I are in complete agreement. There's a large proportion of those addicted to all sorts of drugs that go hand in hand with mental illness. People will self-medicate for mental illness.
This is a different issue—the issue that if you try marijuana, it's the gateway drug to harder things. There are many studies that show correlation; however, there's a difference between correlation and cause and effect. This has been studied for decades. Every time they saw a correlation, there were so many confounding factors and comorbidities that it was never definitively established that marijuana was a gateway drug. To the best of my best knowledge of the literature, that particular point still hasn't been firmly established.