"Protein is not generally going to be used to an extensive degree for fuel. Some is needed to be converted to Glucose for those cells that can't use Ketones. But, most gets used to build new tissue, and for other needs that the body might have for it...and what is left over is not very easily converted and stored as fat, so Ketosis/Lipolysis is a very inefficient state, making it ideal for someone who wants to lose Weight/Fat."

Technically (if one can say that about the human body), one doesn't need to use protein based gluconeogenesis to get the required glucose for operating anaerobic cells. Reforming anaerobically burned glucose back into glucose means all anaerobic energy can be "recycled" "infinitely" (not exactly, but you get my drift). The glucose that gets burned can be formed from the glyserol-base of fat burned for energy. According to one biochemistry text, the numbers came down to 19grams from fat glyserol and 16grams of glucose burned, so given a bit of leeway I'd say that comes suspiciously close to being evolutionarily purposeful: barring refueling after anaerobic bursts of excersice, the human body is totally fuelable by fat.

Go fat! Fat rules! :P