Not harmless and neither is alcohol. But what is harmful is when it is conceived in the same category as other illegal drugs that are a lot worse for you than cannabis. Decrimilisation of weed would, IMO, widen the gap of a progression into more harmful crap like coke, smack, Pringles, heroine etc. (Kids getting ****ed on alcopops are not as likely to go straight up to a line of cocaine, whereas once they've tried an illegal joint of weed, they may be urged to try something else illegal)