I think the real issue here with MM is that it's not really the end game, is it? We don't prescribe opium to people to relieve their pain (or stop a really bad cough for that matter), we prescribe drugs distilled from opium that bring out the components that relax muscles and make the brain process pain differently while (hopefully) eliminating some of the components that increase addiction (this has not been terribly successful but still). We don't prescribe birch bark for headaches, we prescribe aspirin. And once marijuana is studied in depth, we'll likely prescribe some kind of pill or something that hones in on the exact chemicals that reduce nausea or what have you. *Not* a dime bag, *not* a special brownie, a few pills, which by the way might themselves continue to stay very, very controlled even if marijuana is made legal down the line (I think of codeine as a cough syrup, which you absolutely can *not* buy over the counter because, well, it's still close enough to actual opium that it can make you reeeeeeeeeeeeeeally happy if you want it to).
(As an aside I find it *extremely* unlikely that when the time comes to where THC or whatever the cocktail of ingredients used in the marijuana-type drug come in, that it'll continue to be something that you smoke. Smoking is a pretty bad lung irritant already - just the act of smoking might be enough to cause cancer, no matter what it is you're putting into your lungs - and what's more, it's an *extremely* inefficient method of drug delivery. If memory serves, if you take the tobacco from one cigarette and instead of smoking it you unroll it, dump it into a cup of hot water, and let it steep for an hour or so, the resulting tea will contain enough nicotine to kill the average human something like 3 times over. You will also not be able to take more than like a sip of it without gagging*, but in some alternative universe if you were able to get all of that down... well, you wouldn't be able to get all of it down because you would die a horrible death part of the way through. The moral of this story isn't "don't make tobacco tea" - that should really go without saying - it's to highlight how amazingly *bad* smoking is at getting chemicals into your body).
I have to say that I really, really don't like the "back door" aspect of medical marijuana. I agree with everyone in here who says that marijuana ought to be legal. It's pretty mild compared to a lot of drugs out there, including alcohol, it doesn't, like, cause psychosis in a massive proportion of its users like some drugs do, and, well, when it comes to this stuff I tend to err on the side of allowing individuals rather than the government to decide if something is right for them or not. That being said, there are many ways that the medical angle could hurt legalization rather than help it. First and foremost, what if people figure out that there are better nausea suppressants / etc. out there that don't cause a side effect of (see potential side effect in as-yet-undiscovered concoction derived from marijuana)? That's basically the story of cocaine, which went from being used fairly widely as a stimulant, a means of combatting toothaches, and (ironically) a way of getting people off of opiate addiction to basically being completely unused by the medical community at all once they figured out that cocaine is itself really addictive and there are other things out there that do the tasks that cocaine performed better. If anything, exploring the medical uses helped *cause* the substance to be more restricted - at one point in time, Coca Cola was called what it was called because it contained cocaine (well, coca leaf extract but it amounts to the same thing)!
The Pandora's Box issue is a pretty big one IMO as a persuasive tool but the core issue for me is just that marijuana used for recreational purposes is just not marijuana / its subcomponents used for medicinal purposes and you just plain should not combine support for the one with the other. They're not the same thing, period. FWIW I think there's actually a pretty decent argument to be made for opium - and I don't mean heroin or codeine here, I mean straight up opium poppies - to also be legalized for recreational purposes but we can't have that conversation in large part because people decided long ago to link recreational usage with medicinal.
*Ughhhhhhhh don't ask me how I know this! Let's just say that my dad, for a time, took to using old Starbuck's cups to dump his ends into, and, well, sometimes when it's like 6 in the morning and you have been awake all night because you worked the night shift at the radio station all weekend and you still have to go to class for 8 hours, it's hard to tell which cup is your dad's faux ashtray and which is the cup you just bought on your way home to grab your books. WELP I GUESS I JUST TOLD YOU ANYWAY