First off, what convinces me that most people don't really care about the medicinal benefits of marijuana is if you put this out for them to consider...
if marijuana becomes legal, you're gonna get legitimate companies that have to follow government guidelines for marijuana. Growing will probably still be illegal over a certain volume. On top of that, it will probably be subject to a boatload of taxes. End result, that gram of bud or oz of schwag will prolly balloon in price 2-3 hundred percent since the demand for legal weed appears to be so high(no pun intended). I find the majority of people who think on this tend to become quiet.
Otherwise...Marijuana is illegal because it is a mood altering substance. You can say, so is alcohol and you'd be right. The kicker is, you are not allowed to vend alcohol without a license. Public intoxication is against the law and can be interpreted, depending on where you are, as simply drinking in the park. Even having an open alcohol container is prosecutable. The same goes for marijuana. You should have it or consume it in public...and there is no way you can claim that 10lb bag in your closet is for personal consumption rationally, but you are ok so long as you don't have baggies or a scale. If you get caught with a pallet of smirnoff in your house tho, you are damned sure gonna have to explain that. You can claim that cigarettes are also mood altering substances...and you'd be right...but have you noticed you can't legally smoke a cigarette anywhere these days? You can't smoke in the streets really, not in bars or restaurants anymore...plus you aren't allowed to go around selling stoges to peeps on the street. On top of that...should you refuse to put out your cig when someone asks you to? God help you in the country where you can be sued for selling coffee that is too hot.