If we take for granted that there is a true metaethical theory, then it depends on what that metaethical theory says. Unlike Elizier, I would argue that there are plenty of possible metaethical theories that would at least arguably override subjective opinion. Two examples are the Will of God metaethical theory (if an omnipotent God existed) or the Purpose theory (which states that although humans are free-willed, some actions do or do not contribute to achieving a human's natural purpose in life. Said purpose is meant to be coherent, unlike evolutionary purpose- so better achievement would lead to achieving satisfaction in the long run). These are debatable, but make ethics one way or another more than mere human opinion.
Without any rational evidence moral nihilism cannot be considered refuted. Under Elizier's theory, moral nihilism is refuted in a sense- but without a rational argument to oppose it, the metaethicist has no answer. I was a moral nihilist until I read and understood the Sequences, for example.
Finally, metaethics is useful in one particular scenario- the ethical dilemna. When there is a conflict between two desires both which feel like they have some claim to moral rightness, correct metaethics is essential to sort out what best to do.
None of this helps with acting more selflessly and less selfishly, or deciding to do what is right againt selfish instincts. However, that's not what it needs to do.
although humans are free-willed, some actions do or do not contribute to achieving a human's natural purpose in life. Said purpose is meant to be coherent, unlike evolutionary purpose- so better achievement would lead to achieving satisfaction in the long run
If I understand you correctly, your claim that if this turns out to be true, then I ought to perform those acts which contribute to achieving my natural purpose, whether I net-value satisfaction or not. Yes?
...When there is a conflict between two desires both which feel like they have some claim to m
There seems to be a widespread impression that the metaethics sequence was not very successful as an explanation of Eliezer Yudkowsky's views. It even says so on the wiki. And frankly, I'm puzzled by this... hence the "apparently" in this post's title. When I read the metaethics sequence, it seemed to make perfect sense to me. I can think of a couple things that may have made me different from the average OB/LW reader in this regard: