Marijuana is a fairly harmless drug for the majority of people, and in fact is very helpful for a number of medical issues, both mental and physical. Historically, it was only banned because of outside influence rather than actual negative effects. Legalizing it regulates it so that it can be as safe as possible and brings in extra tax revenue.
Legalizing marijuana would be a great boost to the economy. You could tax it at higher rates and once it is legal also create harsher laws that would prevent people from driving while under the influence. As well it would free up some prison space and lessen the load on the legal system (ultimately saving more money) since there would be less people in the court system and in jail for minor offences.
Yes, it is my opinion that legalizing marijuana is justifiable because it has less negative effects on society than alcohol and nicotine, which are already legal. Studies have found that users of marijuana are less likely to commit violent crimes, or cause life-endangering accidents, than people who abuse alcohol. Alongside marijuana's applications as a pain killer, especially in the treatment of terminal patients, I believe this makes it justifiable to be legalized.
I think any drug that messes with the brain should not be legal. I know the arguments of marijuana and alcohol being similar, but I don't think it's justifiable. I wouldn't want people driving around on marijuana. We have enough problems with drunk driving in the United States. It's just not worth it.