The "limited value" isn't so limited when we're talking about an additive to gasoline. The first thing we tried polluted the entire world with a background level of lead!
I have no idea about the US, but in Europe it's absolutely not the case. We've replaced huge quantities of land that was twenty/thirty years ago dedicated to other crops.
Also, we could actually convert them to pastures, that have a much better ecosystemic value (or even let them grow into unexploited forests, for even better environmental effect).
> They can't grow better crops without massive works of engineering and tons more fertilizer and energy use.
Most crops in the modern world run an engineered soil anyway.
In fact, in Europe the most fertile soils have long been destroyed by urbanization (because they were where the population density was the highest in agrarian times and where the megalopolis arose).
> The "limited value" isn't so limited when we're talking about an additive to gasoline. The first thing we tried polluted the entire world with a background level of lead!
We only got there because it was promoted by denying scientific evidences for many decades. Diesel engines have their own issues but they don't require these additives and you cannot pretend they don't exist.
> Most crops in the modern world run an engineered soil anyway.
What do you mean by engineered? The most fertile places in the US (i.e. the southwest) run on multi-million year old alluvial plains where micronutrients are deposited from mountain runoff. NPK and some micronutrients are supplemented but the most fertile regions tend to be the least "engineered". The engineering goes into the massive irrigation projects, not the soil, precisely because engineering the latter is so much harder.
In a pasture for instance, grass can grow because the plant incorporate enough organic matter in the soil to be consumed by microorganisms that will in return fixate the nitrogen from the air into nitrates that can be consumed by the plant. Then you have some equilibrium-ish (it depends on the seasons and the precipitation so it's not an actual equilibrium) amount of nitrogen and organic compounds in your soil.
When you plow the soil, you accelerate decomposition of organic matter that was previously sitting there (because you bring excess oxygen). In the short term, it favors the fixation of nitrogen by the microorganism of the soil (which is why fallow works) but the following years you have less nitrogen fixation than you'd have had otherwise (because there's less organic mater to provide energy to the microorganisms).
Enters the nitrogen fertilizer: with them you don't need microorganisms to provide the nitrogen for your plants, and as such you don't care about the organic matter load of your soil. That's what I call “engineered soil” in opposition to the soils that are driven by the microorganisms who balance the carbon/nitrogen content of the soil.
Of course that doesn't mean that the whole content of the soil is man-made, but coupled with other fertilization methods (which bring nutrients that were naturally almost absent from the soil before), it helped transformed regions which used to be margins with very low yields, into agricultural powerhouses (For instance, Brittany, the region I'm from in France, went from being one of the poorest due to low soil fertility, to the agricultural leader of the country).
The same way, humans have engineered forests since prehistory, but there's still a massive difference between a prehistoric forest and a modern exploited one.
We poisoned the world with lead because it was more profitable for a single company.