Only when idiots are doing it and using edible grains.
When done with the INEDIBLE portion of the crop (which is much bigger than the edible portion), and other biological wastes such as manure and animal renderings, it is a damned good idea and will become economical as soon as petroleum hits the $80 to $100 per barrel range.
Anything Into Oil
Technological savvy could turn 600 million tons of turkey guts and other waste into 4 billion barrels of light Texas crude each year