So you are advocating the continued illegality of marijuana based on what someone "might" do?
Is this an argument that you would abandon if I were to say that using your logic "guns" should be illegal?
Because, as we have seen countless times, someone gets enraged with his or her spouse and in a fit of anger and poor judgment they "might" pick up said gun and use it to kill said spouse?
Now I am in no way advocating making guns illegal. I am simply trying to point out the weak nature of your argument.