Note that the "medicinal" use of alcohol — "self-medicating" — is considered especially bad. The good alcohol use is for personal pleasure — one might say the pursuit of happiness — and most certainly not out of a physical need. Isn't it odd that it's the other way around for marijuana?
If you watch the whole segment — 16+ minutes long, sorry — there's a lot of discussion of the way the federalism problem would work out if the state stopped criminalizing marijuana. It's a misnomer to say that would "legalize" marijuana, because the federal crimes still apply. It would still be a crime to possess, grow, or distribute marijuana. The federal government can't force state officials to carry out the enforcement of the federal law. (It can lure them into that role with conditions on spending, but it can't commandeer the state law enforcement personnel. That's Printz.)
As Matt notes, Eric Holder announced a few weeks ago, that if Prop 19 passed, federal drug agents would "vigorously enforce" the federal law in California. California's a huge state, and that would be damned hard to do. I suggest that Holder may have only said that to try to influence California voters to reject Prop 19, and Matt seems certain that was the reason. And that seemed to work.
It would have been quite chaotic if Prop 19 had gone the other way. In the 16+ minute clip, you can see that Matt loves the idea of the chaos that would destabilize everything with, perhaps, the ultimate result that the federal government would give up on its marijuana crimes. I, on the other side, resist the chaos. I don't think it would work well to have something appear to be legal and at the same time be a very real federal crime. Much as I like decentralized law and the benefits of federalism, where there is valid federal law, it supersedes state and local law. That is the constitutional structure.