While I think that smoked marijuana is not a good medicine (it's exponentially better to press the juice of the fresh plant), there is no reason on earth it should be illegal.
Marijuana a "gateway" drug? What do you think alcohol and tobacco are? Try to find a junkie who doesn't also do one or both.
Marijuana bad for health? Compared to alcohol which is at the root of an alarmingly high percentage of violence and deaths on the road.
Marijuana law enforcement is hypocrisy to the nth degree. It's a big business for cops, lawyers, judges and jailers - a huge percentage of whom are substance abusers themselves (steroids, alcohol, cocaine, and even marijuana itself.)
It's also a form of social control.
Need a reason to hassle otherwise law abiding people? "Drugs" give the excuse.