Sorry for writing an essay... I wanted to be thorough from my medical perspective. Skim if you wish!
If all drugs were legalized, there would be an initial systemic shock of abuses, I think. Addiction rates would skyrocket with the more addictive ones, though in theory supply and demand would create shortages and so maybe it wouldn't be so widespread. In any case, the rates would taper off. In my view, something illegal is more tempting to try. Most people start experimenting with drugs in their late teens to early 20's, partially because this is the rebellious phase (for lack of a better term). Rebellion speaks to doing what is not allowed.
My main concern is that most people don't understand the original uses of the drugs, and just want to abuse them. For instance, many of the organic varieties such as cannabis (also known under its political name as "marijuana"), mushrooms, peyote, etc. all originally had spiritual uses among indigenous cultures. They were (and still are) used as methods of connecting with a sense of higher power, for analyzing one's deeper internal processes, and for exorcising the mind of troubles. Maybe legalization would allow further education, and in turn people would discover appropriate uses from the original sources.
Ecstacy was originally made to help with panic attacks. The active ingredient, MDMA, when in pure form, is less harmful to the adrenal system than ecstacy. Ecstacy can be cut with a number of things, such as speed, DMT, even poisons, in order to increase profit while reducing use of the key ingredient MDMA. MDMA has a lesser duration and does not tax the nervous system as intensely. Crystal meth, if I recall, was first made by the Japanese for their suicide pilots, but even the allies used it to increase soldiers' endurance and treat battlefield depression. It comes from ephedrine which, in most drug stores, can still be bought on the shelf. (Except in areas that are trying to control meth production, in which case it's behind the counter.)
Cocaine.... I don't see much systemic worth to this refined derivative, but its original source, the coca plant, has a wide array of medicinal uses that I support. It's a beneficial treatment for arthritis, and a wonderful calmative for those suffering from chronic pain and post-operative pain. Cocaine itself does have some medical uses, but I would consider its presence in any medical facility to be high risk.
Heroin... this is a tricky one. Opiates do have beneficial uses in medicine, but for recreational use I don't really see the point. I would support every person in the world having a small amount of smokable and/or ingestable opium in their home for medicinal use only, but because it has been illegalized for so long, social policy would prevent people from using it responsibility. For instance, in China, prior to the Opium War with the West, opium was considered medicine and addiction was rare.
Cannabis... I find all smoking offensive generally. Your lungs are not meant to accommodate combustable material, and no matter what way activists spin it, it's unhealthy to inhale any kind of smoke. It damages the cilia of the lungs and renders them more susceptible to infection. That said, cannabis is comparatively better than most other smokables. It does not cause cancer, has huge medicinal potential, and has non-offensive recreational uses. Out of all controlled substances this is one that does not belong on the list.
Tobacco - and by this I mean pure, organic tobacco, straight from the plant, dried, and rolled into smokable form or put into a pipe, not industrially refined tobacco - also has uses. Again, I find smoking offensive, but if I am to be honest I cannot omit the benefits. The nicotine in tobacco increases thoughtfulness and concentration, which is why it is passed around at a lot of indigenous group gatherings because it facilitates open conversation. While I have not had the chance to read studies comparing industrially produced tobacco with organic varieties, I'm willing to bet that the latter is less carcinogenic.
Alcohol... depends on what kind. Moderation is key, but it also has addictive potential. I believe low percentage alcohols are beneficial to blood chemistry as well as the cardiovascular system, especially when had with meals. Getting drunk to the point where you are throwing up is essentially poisonous to the liver and brain, and I do not support it. I am of the mind that alcohol is one of the worst offenders to society that has been allowed legal status, but I blame the user and not the drug itself.
In the end... it's up to the user to define what is appropriate to them and the rest of society. In any debate that involves alcohol, for example, you can't blame the substance for the occurrence of drunk driving, or bar fights, or any number of negative happenings. The same goes for the other drugs. With the exception of a few (like crystal meth), I don't believe a drug is inherently good or bad. It is based on the intentions of the user.