The reason it is illegal in the first place is due to the counter culture revolution that took place in the late 1960's. The government classified drugs on a score card and then passed it as a blanked law instead of passing laws for each individual drug that, it was felt, was contributing to the 'moral decay of America's youth'.
Prior to this 'civil disobedience', it's legality was not a question.
As with any drug, benefits of marijuana (clinical studies reducing the chances of prostate cancer, assisting with chronic pain, appetite enhancement for chemo patients) and risks ( lung cancer, possible addiction, thresh-hold usage) must be weighed, and ultimately, the decision to use it or not (legal or illegal) is up to the individual.
There seems to be a majority, when voted upon, who feel that: 1) the punishment for personal use is excessive to the crime itself, and, 2) The addictive qualities are marginal as compared to other drugs, legal (alcohol) or illegal (Opiates, Coca, etc.). Then there is the monetary gains issue (taxes, licensing, drug court, etc.).
According to national political polls the largest voting age groups are 40 and older. It makes one wonder.