"chronic pains" or using it for "stress relief".
And while marijuana can be used medicinally, it can also go way the other direction. Marijuana causes the heart rate to shoot up between 20% and 100%--making marijuana smokers 5 times as likely to have a heart attack right after smoking. In younger smokers, it can stunt basic emotional development and cause paranoia in their still-growing brains. And if you have an anxiety disorder, marijuana may actually cause a panic attacks. Overall, marijuana users are more likely to get sick and miss work than non-users. And finally, it hasn't been proven, but it's been suggested that marijuana may causes schizophrenia in young users. Not to mention, smoking in general is bad for you--it deteriorates lung tissue.
I can link to things, too.
Here's the heart rate study. Here's the panic attack one. This is the one about paranoia. The one regarding illness. Here's the one that contends marijuana causes schizophrenia.
And yes, black market distribution of marijuana will go down, but it won't stop altogether. The proposed tax on marijuana (the one that would "fix" the economy) is $50. You know as well as I do that people won't want to pay $50 to get high, so there will still be black market sales of marijuana. And just because someone is non-violent doesn't mean they aren't a criminal. That's ridiculous. Prisons are full of robbers, embezzlers, etc. Do you remember Bernard Madoff? How about Rod Blagojevich? Neither of them committed acts of violence, but do you consider them criminals?
Ok, even if coming up with new regulations doesn't take long, it will still be a pain to enforce. It's easier for the DEA when everything about marijuana falls under the umbrella of "illegal". If heavier types of marijuana are illegal, etc, the DEA has a harder time because it has to investigate, test the sample, and more before it can tell if the weed is illegal or not. So it won't save time for the DEA.