I have to jump in here real quick and disagree with eav. Cannabis, in its day of widespread usage, was practically a wonder crop, and it still would be if it were in use today. You can make very high quality rope, clothing, and general consumer materials such as backpacks, wallets, belts, shoelaces, hats, and virtually anything else that can be made out of leather, cotton, or similar materials, out of hemp, a product derived from the stem of the cannabis plant. Even better, all of it is EXTREMELY durable. You can also make paper out of hemp, at the same quality as paper made from trees. The key difference there, though, is that you can get four times the amount of paper from an acre of cannabis that you can from an acre of trees, and the cannabis plants will return at full size the next year, rather than taking many, many years to grow back like trees do. And all of that is just out of the plant's stem. The cannabis plant also yields edible seeds similar to poppy seeds, and the oil from the plant can be made into a cooking oil that makes a fine substitute for vegetable oil.

All of those products are, of course, in addition to the product of the cannabis plant that we all know about already: marijuana. While the value of marijuana in an open, legal market is debatable, I think it would do at least somewhat well. It should be noted, however, that cannabis plants that do not have the main narcotic ingredient of marijuana (THC) in them are rather easy to produce, and I believe Canada already produces these non-marijuana cannabis plants in some regions.

So as you can see, cannabis would still be a major economic threat if it were legalized in the United States. It is a weed that can be grown in anyone's backyard, but because of the industrial processes involved in turning cannabis into the majority of its valuable products, it would still be a crop mainly produced by farmers. Those farmers, being a necessity to create hemp and legitimate consumer products like hemp oil, hemp seeds, and pre-packaged consumer cannabis, would benefit from the plant's hardy constitution. Since it can be grown in almost anyone's backyard all across the US, it can also be grown in almost any farming environment as a main or secondary crop, and its normally insidious nature as a weed would ensure that it would return every year with a fresh crop that most likely wouldn't be damaged by adverse weather conditions. The farmers would also benefit from the range of products that cannabis can be turned into. With a place in such a diversity of markets, there would be a huge demand for cannabis from companies that want to turn it into their consumer product of choice. A demand so large that it would probably be able to satisfy any farmer that ever wanted to get into hemp farming.

Cannabis is extremely easy to grow and can be turned into an incredibly wide variety of products. Even now, it would quickly dethrone the current market leaders in paper, clothing, legal narcotics, and several other markets if it were legalized. The reasons for keeping cannabis illegal are no longer mostly economic, but if politicians and the general populace ever went past their political and moral objections to cannabis (however right or wrong they might be, which isn't the point), the economic reasons would surely pop up and would probably put a stop to any attempt to broadly legalize cannabis.