I've written much longer posts about this in the past and I'm not going to dig them up or write anything of length today, although some of the older posters here (GhostAnime?) may remember. But here's my few thoughts.
As much as I think the illegality of marijuana and of drugs in general is destructive, one thing that sure ticks me off is the pure ignorance that perpetuates in most of the pro-marijuana lobby. Smoking dope is perceived as some sort of soft drug and almost harmless by far too many people, including almost all of the lobby. Unfortunately for the many who smoke it, that's simply not the case.
Basically there is a lot of very good evidence linking marijuana to early onset psychosis and schizophrenia. It's both clearly visible if you do any work in mental health wards (a twenty year old with psychosis is a sad sight indeed, and its almost always correlated to marijuana), and more and more visible in the number of statistical and meta-analytical studies into the issue. A landmark paper in the Lancet from a few years ago springs to mind (unfortunately I don't have time to dig it up, I'm sure people who have debated this before would know of it though). This doesn't prove causation obviously and there are some strong arguments that THC merely lowers the threshold for psychosis (and thus unleashes it in those prone to it anyway) and this is something the smarter elements of the pro-cannabis lobby have lept onto, but let's consider what this actually means. It means that instead of having a possibility of developing psychosis when you're forty or fifty, you're going to get it when you're twenty after a year of getting high every weekend. I don't think many people who support the wider use of cannabis really understand fully what this would mean: schizophrenia is an incredibly disabling and debilitating disorder. If you develop it in your twenties your entire life is essentially blown to pieces: even if you make a recovery, from there it'd be almost impossible to be successful in your career, establish a family, do the things that in the long term you'd probably want to do with your life. From this point of view, marijuana is far more dangerous drug in far more subtle ways than cigarettes and alcohol.
I'm not advocating for the full prohibition of marijuana. Personally I think it should be decriminalised, although I would want to see its sale remain illegal; if you take drugs its one thing but as far as I'm concerned dealers are the scum of the earth. People should have the ability to make the choice of what they want to do with their lives, and smoking pot is one of those choices. Unfortunately it is an incredibly bad choice, and a choice that many of those who now advocate it in a few years will come to regret.
As the debate on reefer has devolved to It's good/it's not good, an anecdote may just be whats needed to spark the topic back into a discussion rather than yes/no.