The cancer is caused by the act of smoking not the marijuana itself. THC is a relatively harmless drug (though to be frank it's hardly a drug it's not addictive and you will not suffer withdrawal if you stop smoking it) if it were say made into a cookie or brownie or some other smoke free version it would be mostly harmless. it doesn't help much i'll agree to that it doesn't exactly benifit productivity or help your food bill but really compared to the things we put in ourselves it's much MUCH worse than many many things that are legal and widely considered "not that bad".