"In another study of seriously injured individuals admitted to a Level-1 shock trauma center, more than a quarter (26.9 percent) tested positive for marijuana. In a study of fatally injured drivers in Washington State, 12.7 percent tested positive for marijuana." -Dr. Robert DuPont, former pres of Institute of Behavior and Health
He also claims that the usage rate in 2008 was 6.1%. That shows a highly disproportionate % of weed users in fatal accidents and serious injuries. I find it hard to believe that those users were the only victims in the car accidents/other incidents. So i don't think you can claim that marijuana use does not impair one's ability to function nor that it is a harmless drug. It sheds doubt on the claim that it is victimless.
Now, I understand that there is a lot of research that claims the opposite of what I have cited, but my point is that there aren't many hard-and-fast conclusions about marijuana's use, so it is premature to claim that its use poses no risk to society.