i could write a long thought out essay on this, which i would really like to do, but instead ill write an abstract of sorts:
Marijuana can be harmful if used incorrectly. With overuse, it can lead to addiction and, as with any smoke, cause permanent lung damage. However, the benefit (getting high) normally outweighs the cost (monetary and health). The government spends billions of dollars per year attempting to control the supply of illegal drugs, dollars which could be spent on education, healthcare, etc. Also, with the illegal nature of drugs and drug dealing there is associated violence due to money conflict. If I decide I don't want to pay for my weed, the dealer isn't gonna go to the police and say I stole from him, but will probably have me fucked up pretty bad. If it were legal, we could control its sale and distribution to avoid these violent conflicts. More effort could be spent on education to encourage responsible use, rather than simply telling kids not to do it. When kids don't know how to use it responsibly, they end up as the "stereotypical pothead" and smoke far too often. Marijuana is an excellent way to relieve stress, have fun, and is also useful for meditation and introspection. Again, responsiblility is the key. Just as we are to be responsible users, the Government is responsible for our health. By legalizing marijuana, they can eliminate the associated violent crimes and encourage more responsible use.
some facts:
Half of the U.S. prison population is incarcerated on drug-related charges.
Upon repealing prohibition, the crime rate (excluding alcohol violations) decreased significantly
The demand for drugs will never decrease no matter how high the price is, so supply controls (border guarding, busting dealers, etc.) will never cause a decrease in usage.