Why would it be unethical? Is it because we would be unhappy if we were servants? Consider how some people are happy to work in places that most of us would be incredibly unhappy working in. They choose it. They enjoy it. Perhaps their brain is wired that way, differently from ours. Is it unethical to employ them? Consider--Dogs have been bred to enjoy serving humans at menial or dangerous tasks for millenia. No one (well, almost no one) considers the use of dogs as working animals to be unethical. If they were deliberately created to enjoy unpleasant tasks, why would it be unethical to employ them for such tasks? They would not consider themselves to be suffering, so why should we?