As I see it there are a few different stances towards drugs.
1) There are people who disagree with drugs, who would never take drugs, and believe that there is no need to take drugs.
2) There are people who take drugs and believe they should be made more readily available without fear of legal reprisals.
3) There are people who may or may not take drugs, but who understand potential benefits of decriminalisation.
4) There are people who may or may not take drugs, but who disagree with decriminalisation.
Those in group 1, I believe, are naive. They disagree with taking drugs and criticise those that do. Clearly these people would have no personal experience to base their opinions on as they are so strongly positioned against drugs. Their information must therefore be obtained via a mainly sensationalist media who, for the most part, are anti-drugs. It isn't hard to find (reputable) sources that will argue that currently legal drugs (alcohol and tobacco) are more dangerous than the likes of ecstasy and cannabis - David Nutt springs to mind. Will those people in group 1 take interest in this information? Unlikely, as it isn't in their interests as they have no interest in drugs anyway.
Those people in group 2, whilst I agree with them, are really just looking out for themselves. They will find it hard to persuade anyone in group 1 that drugs are perfectly acceptable as the crux of their argument will probably revolve around explaining how good ecstasy makes you feel.
I'm in group 3. There is a strong argument to support the decriminalisation of drugs for a number of reasons. Firstly, there is the aforementioned evidence that shows currently illegal drugs to be relatively harmless when compared to legal drugs. Secondly, the decriminalisation of drugs would potentially reduce the amount of associated crime whilst consequently improving the quality and safety of the drugs available to people. Take MDMA - barring allergic reactions it won't kill you unless cut with other incredibly more harmful drugs. The only way to ensure it is good quality is to have some sort of legal regulation in place. Thirdly, as in Portugal, it might actually help drug addicts.
Also to Klo - everyone who takes drugs isn't missing something in their lives. If a substance is there that can safely enhance an experience then I see no reason not to take it.