Better question. Who's promoting it to us? The government bans drugs so that the rebellious waste their time poisoning themselves, instead of actually taking any kind of action. They push drugs into the realm of "counterculture" so that people will rebel and use them instead of channeling their feelings into anything productive.
Weed may be an entirely different situation, given that it is virtually harmless. Sure, it's a slight irritant, and it's a distraction, but it does have proven medical benefits. I'm not inclined to trust people, but I do believe that it possesses certain medical benifits. If marijuana helps sufferers of Alzheimer's Disease, there's nothing wrong with them using it. Also, recreationally speaking, though it's a waste of time, I don't have a problem with someone using it responsibly and occasionally.
My concern with the whole medical marijuana thing is that people will continue to push the boundaries of what is and is not medicine. Eventually, we will be medicalizing actual poisonous drugs. I think the limit should be drawn at derivatives. Processed derivatives like opiates should be prescription only, and straight plants like marijuana should be completely legal for all of the adult population. The exception being tobacco, in which case tobacco companies should be required to use pictures of cancerous lungs to "promote" their products.