A - MJ, while a mild drug, is still a drug, and it seems like people who use it everyday are addicts.
B - On the one hand I'm OK with people making their own decisions as to how they want to live their life, but on the other hand if we are talking about the type of society that we want to have, lots of people taking hallucinogens to get through the day doesn't exactly fit the image. Something seems wrong.
C - We don't really know the long term impacts of doing this. It is probably somewhat bad somehow, even if we don't know exactly how.