The general argument against legalising psychotropic drugs is simply that it will cause everyone to become a junkie. This is akin to saying that legalising homosexuality will cause everyone to have gay sex. Drugs appeal to very specific segments of the population - segments which would probably get drugs anyway, but if drugs are not legalised would both pay more for those drugs, and would pay for diluted harmful versions of those drugs. In Shameless, Frank Gallagher says something like "Make poverty history, cheaper drugs now!". Would junkies really get sent to jail for robbery if they simply had enough money to pay for their fix? Well, they probably would attempt robbery anyway, simply because the money is just going to run out slower if drugs are cheaper. But still, the rehab clinics that actually provide heroin could take care of that. Some people simply can't maintain a job and a drug habit at the same time.
Another argument is that drugs cause harm, and we should stop them. By violating our rights to do what we like with our bodies* and by putting people in jail who have committed victimless crimes - possibly whilst trying to help solve their own pain at how the world works? Another reason why this simply doesn't work as an argument is because reality gives the finger to it. People get drugs regardless of whether they are legal or not. If there is a demand, there is a supply. But IF you make it illegal, the drug quality decreases, causing EVEN MORE HARM than if you had never made them illegal in the first place.
A final argument is simply that people should live longer and more healthily. I disagree with that, as someone who vehemently hates his life, but still, if that were true, we'd do a lot more campaigning than simply making KFC illegal along with drugs. People will get what they want eventually. Put barriers in the way and you only delay things a bit.
What would drug legalisation bring that would benefit humanity, you ask? Well for one, a lot of research into drugs is blocked simply because they are illegal, so any good effect that might have is often cast aside. More drugs being legal would make alcohol less popular (maybe), and alcohol, as you should know, often is the cause of embarrassing things (along with violence, a lot of violence). Marijuana on the other hand, as a social drug, does not cause these things, though driving stoned is still going to kill people. Lastly, we could finally start work on David Pearce's dream. Making people far, far happier than they are by providing them with a limitless supply of opiates and other such drugs - tailor-suited to reduce tolerance and such things (if that's possible). Drugs, for the future (TM).
*Tried SO hard their not to imply that we 'own our bodies', Franc. The English language doesn't lend itself well to me for doing such a thing. See The Confusion of 'Self-Ownership'.