People who think smoking pot is harmless are wrong on so many levels. Why do we need another bad thing to be legal?
Separately, marijuana being illegal while smoking, alcohol, and surgery foods are legal is a contradiction. If you want the government to regulate everyone's lives for the greater good, you might as well ban those as well.
I'll give you this, though: in many conversations weed is being made out to be totally benign. While I think it's more benign than alcohol or many prescription drugs, it still makes people lazy, contributes to poor health, can be carcinogenic if combusted, and can be abused in the sense that a user can rely on it to attain a sense of exhilaration or happiness.