Most people who feel it shouldn't have been made illegal in the first place, feel it was racism, fear and hate basically, that got it made illegal in the US in the 30's. A basic intolerance of things different, I suppose. Either way, the 'war on drugs', which is really a war on people, costs a silly amount of money to keep going each year, and there is more pot available now than ever before. As stated earlier, it hurts all law enforcement to have laws on the books that tens of millions of people ignore and work to circumvent. I should say that I do think the tide is turning though, and the majority of the laws that matter will be changing in the near future.
Anyway, since the original topic was legalizing as a means of stopping drug violence (presumably along the border with Mexico), I'll throw out this brief being made available to the California legislature, as they prepare to discuss and vote on AB390.
http://www.canorml.org/background/CA_legalization2.html
It's difficult to say in advance exactly what the benefits would be, but this is a fairly thorough look at what might be expected from a state revenue perspective.