1-The drug generally isn't more harmful than alcohol or tobacco if used in moderation
2- Limiting the use of the drug intrudes on personal freedom.
3-Legalization would mean a lower price; thus, related crimes (like theft) would be reduced.
4- It could be a source of additional tax revenues
1-Marijuana is often used as a stepping-stone drug, leading to heroin, cocaine, or other harder drugs.
2-Legalization would increase the chances of the drug falling into the hands of kids.
3- Physical damage would be done to users that abuse the drug.
4- Legalization of marijuana could eventually lead to the legalization of harder drugs or all drugs altogether.
You can't think of marijuana as a steppimg stone drug, if you think like that you wouldn't let your kids out because they could be run over by a car, it's not black and white, it's in a grey moral area, but still it's not a hard drug, and if tobaco and alcohol are legal marijuana must be legal, it's only logical.
If you think about it most governments need the extra revenue and if it is responsibly legalized the is not a single problem that can come in the way.
In the case of Amsterdam, the most famous example, it was a smash hit, marijuana gave a new life to the tourism industry, there are no drug deals, the number of prisoners got reduced and work productivity increased significativly, so with some responsibility it is possible, but it has to start with the people.
I'll just end with a video, you'll be surprised, judge on your own.