The point is, if you want to eat meat, you have to feed both the cow/pig/whatever AND yourself. If you don't have to grow the cow, you can eat the grain.
Of course you still have to do it right, you need to grow more soja beans and stuff. You can probably still eat SOME meat, because you'll need milk and eggs. Those cows and chickens will die when they're old... Yeah.
But let's not start a fight on that in this thread.