It's more complicated than that; goblins don't need to eat partly because that allows them to have cultures that are dissimilar to human ones.
From a question to Toady One:
"Toady do you mean Goblins don't need to eat in any future update or that Goblins won't need to eat until you can get it working properly?"
"In any future update. I know a few people were dismissive of our decision and called it a cop out, but we thought about these situations (including the Mongols) and rejected them. Goblins that herd meat animals are insufficiently scary to us. Goblins that die exclusively violent deaths in great numbers in a potentially vegetationless wasteland are better, and we want to explore a wider variety of possibilities than humans allow -- humans can be Mongols, because the Mongols were human, and we hope to support some human variety eventually. There are beak dog considerations, etc., but those aren't fundamental -- wrangling some critters isn't as image-shattering as having large herds of meat animals ranging over grasslands with goblin ranchers."
How different could culture be if a civilization didn't have to eat? One of the cool things about DF is that while in many games, the "nonhumans" are based off of some mix of historic Earth human cultures, Toady intends for the aliens to be genuinely alien. Humans can *already* be Norse, Aztec, Mongol, Egyptian, and everything else humans have been; what are some cultures that humans *haven't* tried, for one reason or another?
Needing to eat is such a fundamental element of not only human, but all Earth animal, life that we never seem to think about how things would be different if we didn't. And one of the other most significant elements of human culture is the inevitability of aging and death. If an entire society did not need to eat, to drink, and would never age or die naturally... how might they be different?
All of the above is from a social / world building approach; which is more than enough justification in my mind. But from a more gameist viewpoint, having a race of foes that can not only live, but thrive, in otherwise inhospitable and in fact downright evil places makes for more interesting game play. Too many games handwave away the evil legions; if the countryside is that blasted, what feeds them? In DF, the wastes are overrun by goblins, specifically *because* they don't need to eat and don't die except when killed.