I am not an expert on hemp, but someone asked about it for an article in the California Aggie. For what it's worth, here's what I wrote:
A well-known agronomy text, "Principles of Crop Production" classifies hemp as Cannabis sativa, the same species as marijuana. Other sources may consider it a closely-related species. My understanding is that industrial hemp is not something that anyone would want to smoke, but marijuana growing in the middle of a hemp field might be hard to spot. Therefore, opponents of marijuana legalization generally oppose legalization of hemp as well. I suspect that some (not all) of the interest in hemp comes from those who favor legalization of marijuana or who think it would make it easier to grow marijuana. The same text recommends growth in humid climates east of longitude 95W. When it was legal in the US, it was grown mainly in Kentucky and the North Central states.
A key question is which crop uses the least water, energy, and pesticides per ton of fiber. The text gives similar fiber yields for cotton and hemp, but I think cotton yields have increased significantly since the text was written. According to the text, "heavy nitrogen fertilization reduces the strength of the fibers" so hemp might be fertilized less heavily than some other crops. On the other hand, overfertilization can make cotton produce too many leaves and not enough cotton. I understand that fertilizer recommendations for cotton are being revised, and the revised recommendations may call for less nitrogen fertilizer.
Sometimes a crop that is new to a region will have fewer pest problems at first, but I wouldn't necessarily expect hemp to remain pest-free indefinitely. Longer rotations (growing different crops in sequence and not getting back to the first crop for a few years) often help to control soil-borne pests. Therefore, any new crop that grows well in a region and can be grown profitably is of interest. In California, closing of sugar plants has effectively eliminated sugar beets as a profitable crop over much of the state. Therefore, there could be interest in new crops. However, we already have a much greater diversity of crops than most of the midwest. If hemp production is legalized, I would expect much of the production to be in humid climates that currently have few crops to choose from. In parts of Minnesota, for example, a two-year rotation of corn alternating with soybeans is common. If it were legal and profitable to grow hemp there, some farmers might value the pest-control and other benefits of a three-year, corn-hemp-soybean rotation.
Although I don't believe all the hemp hype, I do think the US and California (which is already reasonably diverse), would benefit from growing a wider range of crops. But hemp enthusiasts would have more credibility if they showed some interest in reviving other once-popular crops. Considering only other fiber crops, what about flax, ramie ("the strongest of all vegetable fibers"), or kenaf? Ah, but none of them look like marijuana, do they?