Does the conventional wisdom about ethanol for fuel make economic sense? In general the answer is clearly "No!" If you are a corn or sugar cane producer, the answer is certainly "Yes!" With all economic analysis, measures of success vary among parties.
If one is just a consumer of gasoline in the US, which is mandated by law to have at least 10% ethanol, the answer is no. Ethanol, when blended with gasoline, is less fuel efficient and creates more emissions through incomplete combustion.
Although ethanol had been envisioned to reduce dependence on foreign oil, the opposite has occurred. It takes more fuel to plant, tend, and harvest corn and sugar cane, and then to process it, than the resulting ethanol saves. Additionally, ethanol blended gasoline is more expensive to purchase, and since it is less fuel efficient, it is less efficient to use.
Further, when existing crops such as corn and sugar cane are taken off of the food market to ethanol, the price of those crops goes up, as does the price of food. Higher food prices lead to both local and global insecurity. Is getting less mileage, creating more pollution, and spending more at the grocery store worth it?
From a farmer's point of view, using crops which are often subsidized, even sometimes subsidized to destroy, in a useful manner is good. Arguably, higher prices lead to higher output and that is better for the agriculture sector in general. A strong agricultural sector leads to greater food security.
So who is right? From an economic efficiency point of view, the consumer is right and the farmer wrong. The costs to the consumers far outweigh any potential and specific gains to farmers and the agricultural sector. Additionally, the detrimental impact of increased air pollution also tilts the scale in favor of the consumer.
Clearly, using corn or sugar cane to make ethanol for fuel doesn't make overall economic sense.
Here is some video discussing the issue further: