Most of the above have a point, however a few things were omitted. When Marijuana was illegalized in the 1930s, it was merely a government action to suppress the subculture which was causing trouble (hippies.) Ever since then, it's been on the same shooting range in the war on drugs as heroin, cocaine and MDMA. There was no greater philosophy behind it, they didn't want to 'protect' the individual or 'free' society from this drug - it was merely a reason to throw some key people in jail.
      On the flipside, hippie said that it does nothing to people but slow them down. I beg to differ. The affects I observed in myself and friends included not only thought proccesses turned to syrup but also apathy. We didn't care about getting stuff done, we were chill. The gateway factor is another thing I was starting to think about. Before you start throwing things, hear me out - I used to argue that the gateway drug thing was bullshit, since no one starts with LSD or mushrooms. If you were to ask someone who is in rehab for a 'hard' drug, nearly 100% would have started with marijuana; those who smoke a few joints a month are never questioned or included in the statistics. After quitting, I look around. About ten of my friends smoke/d weed. One has quit after doing only it, one still solely smokes it and is considering doing mushrooms, three do mushrooms, one does coke, three do MDMA and one has done codeine and quit. It just makes people less resistant to other drugs, hell, you're already doing something illegal, why not try something different.
      Legalization, to a large part, would put an end to the gateway drug phenomenon. Kids would no longer be taking an illegal substance, other drugs would be in a whole different category.

      Mat_C: That is one way to look at it. However, in europe there is a lot being done with cannabis in the agricultural sense. Ironically enough, I can't remember the name of the species, but they have raised a cannabis plant with little to no THC. Who's to say that Exxon or the textile industry can't include this in their production process? Especially now that we're running out of oil, cultured cannabis would be a considerable solution.