I may be misintepreting, but I wonder whether Yvain's use of the word "emotivism" here is leading people astray. He doesn't seem to be committing himself to emotivism as a metaethical theory of what it means to say something is good, as much as an empirical claim about most people's moral psychology (that is, what's going on in their brains when they say things like "X is good"). The empirical claim and the normative commitment to utilitarianism don't seem incompatible. (And the empirical claim is one that seems to be backed up by recent work in moral psychology.)