Well, you say people could do it in their free time. In all honesty, do you think people will only do marijuana in their free time? Look at alcohol, it could be a casual drink, but the drunks that kill the innocent people ruin it for everyone. Marijuana is addictive, just like alcohol and cigarettes. It could ruin someone's life through the addiction. ANY kind of addiction is bad. Marijuana truly reduces your reaction rate, although it increases your alertness, I know from experience.

Not to mention Marijuana decreases fertility in women and lowers mens sperm counts and ability to get an erection. It can even permanently cause impotence.

I honestly think that Cigarettes, Alcohol, and Marijuana should all be illegal and have high penalties for abusers. Like I said before, any addiction is bad.