Well, besides the money issue, we could stop putting people in prison for using/ growing/ selling weed. The cartels would no longer be a threat, at least on that end. People would have a viable option to drinking without having to worry about the legal ramifications,which may lead to less drinking and driving, which we all know is a big problem in this country. I honestly don't see any downside to legalizing marijuana.
It's already legal in many places to own small amounts, with a doctors skip. It's a drug. It's therefore going to be regulated by the FDA no matter what. If its an herb, its a food-- same deal. But it has mind-altering properties and thus would be regulated even if legalized. Which like beer will come with standards-- THC content, age of purchase, etc.