The prohibition of marijuana has caused many more problems than the substance itself has been known to.
By regulating it similarly to tobacco and alcohol we will be able to create jobs and earn taxes on it. We will also be able to free up law enforcement officers to do more important things, and eliminate funding in the war on drugs.
The prohibition has brought a rise to gangs and cartels. Regulation of marijuana will remove funding from them and place it back into the economy.
Citizens who are not doing any crimes other than possession or consumption are jailed. This can make them lose their current job, and also can keep them from obtaining a new one in the future.