If a farmer can get twice as much money making biofuels as growing food crops, then according to market forces this will increase the amount of biofuel being produced. Folk go hungry, this will push food prices up, so farmers produce less biofuel and more food, and eventually an equilibrium is reached.
The problem with this is that it takes time to produce food and biofuel, and its also impossible to be exactly sure how much you are going to be able to produce (unless you can 100% accurately predict the weather for a year). Therefore in the process of these “market adjustments” there might be a “shortfall in supply” for a couple of years, which would have serious consequences, (like several million people starving to death).
Food production is too important to mess with.
What I want to see is a farmer making money producing biofuels when he is only using biofuels for the whole process.