Legalization is really starting to hurt marijuana cultivators south of the border. Exactly as everyone said it would based on basic economic principles (and people still refuse to believe for some reason). This is NICE. And by that I mean, TOTALLY NICE. For this very reason, a few years ago when California almost legalized, growers in NorCal panicked.
For us outdoors types, the big problem with continuing prohibition is that growers inside the United States seek out remote areas of national forests for their operations. Outdoors bloggers sometimes run into such operations. This is a major problem for conservation - it erodes the soil, introduces invasives, takes water, and increases the chance of forest fires.
So we can hurt drug cartels, decrease forest fires and preserve our open space by legalizing, which so far has apparently not caused civilization to collapse in Colorado. What are we waiting for again? (For a little Onion video humor on the topic go here.)