Rikkitic: Those are valid points. I think the question (which has never been answered) is whether overall harm would be greater, less, or simply shifted. For example, would more cannabis users means less alcohol abuse? Would they cause less trouble due to lower levels of violence when stoned? Would depriving organised criminals of income from illegal cannabis benefit society overall? And so forth.
I think you could be right with the "shifted" harm - if cannabis was more readily available, perhaps people would use other drugs less. But I'm not convinced that legalising cannabis would do much to reduce criminal income in the long term - there might be a period of adjustment but I imagine they would move into other areas (or further into other areas they're already involved in), e.g. P manufacture, and with the effects of P being much worse that could increase the net societal harm.