The benefits of legalizing outway the "drawbacks". Anyone who argues against that is arguing from a personal political standpoint, not an informed economic(al), administrative or legal standpoint.
Even some of the big German broadcasters like RTL or ARD, whose news policies have always been the outlawing of drugs of any kind, have reported on this in a positive way in the last weeks. They have interviewed several American professors who specialize in criminology, economics, addiction and other fields, that are related to illegal marijuana consumption in the US.
The taxation of legal marijuana (as it is done with alcohol for example) could earn the state of California alone more than one billion dollars in taxes per year. The costs of the pursue of illegal consumption caused by police work, narcotics agents, the penal system etc. is at 1.5 billion dollars a year. This number would be reduced by half if marijuana would actually be legalized, because the control of marijuana consumption would be much easier. You do the math.
On top of that, there are several countries that can serve as "models" for legalization. In The Netherlands for example, you can consume marijuana legally, but you are not allowed to do it in public places (same goes for cigarettes or alcohol). Same thing with your workplace. You can be fired when showing up stoned to work every day. It's the same thing as if someone would show up drunk to work every day. You're not allowed to drive when stoned etc. etc.
And at the same time the marijuana industry created thousands of jobs. Imagine how many jobs it would create in the States.
You can apply the same rules to marijuana that are already applied to alcohol, tobacco or prescription drugs that have a similar effect.
I'm of the opinion, that whoever wants to smoke pot for personal recreation or leisure should be allowed to do so. As long as there are clear laws and regulations for it and as long as you go about it in a responsible way, I have no problem with anyone smoking pot. When a stressed out businessman decides he wants to vacation one week a year in his living room instead of Hawai'i with a bag of marijuana and his car remaining in the driveway, who am I to forbid him that?
Anyone, who gets piss-drunk every friday night, starts a fight and drives his car home later but at the same time argues against marijuana because the lazy potheads don't get up from the couch or get the munchies and drive to McDonalds, is nothing but bigoted and two-faced and should seriously consider revising his egomaniacal world view.
