Right, you want a coherent argument for keeping cannabis illegal. Here goes...
I will freely admit that I have not done the research on the drug, and therefore I can only go by what the common media have reported, and by what I have seen. In addition I am not trying to fight an Alcohol vs Cannabis battle, as I have no desire to do so.
There may admittedly be remarkably few examples of people becoming *addicted* to cannabis, especially in comparison to alcohol or other illegal drugs, but I believe the effects of the drug to be nonetheless serious. For example, the fact that different people can respond to the drug in entirely different ways is something that troubles me greatly. That otherwise intelligent people can be turned into nervous, quivering wrecks through overuse of the drug is something that worries me to a rather large extent. It was something that troubled me when I was at university: I was offered the drug on more than one occasion by my friends, and turned it down every time because I saw what it did to them. OK, those effects were only whilst using, and not prolonged, but I remained concerned for what would become of them if they smoked the stuff on a frequent basis. Perhaps this led to a certain yoghurt-throwing incident...
The reports I have read about people who have smoked a lot of cannabis, and from what I have personally seen, lead me to believe that cannabis should remain illegal, especially as the negative effects of it have been shown to occur - yet none of us knows to whom they will occur. I will, however, say this: I would welcome a derivative of the drug to those whose pain is best relieved by it, but only if it and its effects are properly understood and researched.