I also think that harm is a relative term, I know a guy who started smoking cigarettes when he was 8 years old, has smoked two to three packs a day and is 86 years old, still smoking... Looking at him could you say that cigarettes are not harmful?
Marijuana smoking has to be less than healthy, breathing smoke cannot be good for you, to me the real problem is less than neutral studies and people who don't want to hear their drug of choice is harmful.
The man I mentioned above is convinced cannabis is horrific and anyone who smokes it should be put away for life, this he says while swilling bourbon, taking opiates, and smoking like a frieght train... he is a mean drunk too...
My main worry is people whose lives are ruined by the law when they get caught smoking pot....