Why do we need proof of benefits? Cut the subsidies and quit forcing us to buy it. If it survives on its own then it can be deemed beneficial.
A free market place is the only test needed.
I don't think there are any benefits in this case. But suppose that it really was reducing our dependence on foreign oil and in the process helping defund terrorist supporting states. That would certainly be a goal I could get behind and would support government intervention in the market place to help happen.
If Biofuel was likely to be viable alternative to oil given economies of scale and this program was helping achieve those economies of scale, then again I could support the government intervention. Especially if it meant American jobs vs overseas jobs during a time of high unemployment.
But I think there are better alternatives that have already been proven viable like Nuclear, that I'd rather see the government put it's resources behind. I just think the Biofuel, is not going to be viable unless you go to it 100% like Brazil and even then it's very questionable.