One frequent objection to meta-ethical subjective relativism is that it somehow trivializes ethical discourse by calling ethical statements only subjectively true (i.e. ethical statements are fundamentally only true statements about minds). But why should this sort of subjective relativism trivialize ethics? Implicit in this view is the notion that our emotions are intrinsically irrational and our purely subjective properties intrinsically unimportant. But where is the argument for such a position?
Minds are real: They exist in reality. Emotions and subjective properties are real properties. Real properties are facts, just as that the Earth is roughly spherical and ~8,000 miles in diameter are facts about reality. It is always rational to discuss and account for actual facts. It is the case that our ability to make logical arguments is sometimes impaired when our minds are experiencing certain emotions. But this is a separate issue. Emotions sometimes cause irrational thinking, but that's a different issue than emotions being inherently irrational.
This prejudice is partially due to the fact that professional scientists tend to focus on understanding objective reality—the world outside our minds—because objective reality is much simpler than subjective reality. And, of course, to get a good picture of what is truly objective, one has to subtract out the purely subjective. This is not, however, a metaphysical position; it's just a control, in the same way that a scientist will randomly divide experimental subjects to subtract out effects due to causes other than the one she is studying. It's not that these other causes are unimportant, it's just that it's more efficient to study one cause at a time.
This prejudice is also, I suspect, an artifact of the historical sexist position of attributing "emotionality" to women as a put-down and a justification for their oppression. The idea that women are more emotional—and thus less "rational"—than men is egregious bullshit, flatly contradicted by the thoroughly emotional and almost exclusively male institution of war. (It boggles the mind that anyone could with a straight face call the slaughter of hundreds of thousands or millions of people—one's own countrymen as well as the "enemy"—a dispassionate, logical endeavor.)
I also suspect many hold the notion that our emotions and purely subjective properties, not being directly representative of objective reality, are highly variable. But ordinary observation easily contradicts this notion. Yes, purely subjective properties, not subject to constant feedback from objective reality, do vary more than our prosaic understanding of physical reality. On the other hand, our minds—including the intrinsic properties of our minds—are the product of tens of millions of years of evolution, as well as the product of tens of thousands of years of socialization and acculturation. Our common humanity, our common society and our common culture are not just abstract philosophical or rhetorical positions, they are real effects of real processes.
We are, as a species, quite enamored of objective truth-seeking discourse, just as we are enamored of minded teleological discourse. And for good reason: Objective truth-seeking discourse has been very successful at developing science and technology. Furthermore, because our minds are self-referential, we can apply the mode of objective truth-seeking discourse to rationalize and make consistent our purely subjective beliefs, by treating them "grammatically" as "objects".
MESR denies only that truth-seeking discourse applies to the content of our ethical beliefs. Our ethical beliefs are not about the world outside our minds, they are real properties of our minds. But that's only one mode of discourse, and it's an historically unproductive mode of discourse about ethical content. Every ethical philosopher who has put forth an objective theory of ethics has—amazingly enough—justified his own ethical intuitions (and often the ethical intuitions common to his own cultural and social milieu). Hardly a surprise: What evidentiary basis does he have other than the content of his own ethical intuition?
But why shouldn't we consider the content of our ethical intuition to be objectively true a priori? After all, we take the content of our perception to be true a priori; that's the foundation of science, n'est pas? Au contraire! The veracity of our perception—the truth of the content of our perception—is a conclusion, not an a priori metaphysical position. It just so happens that we are so practiced at drawing consistent, predictive conclusions from our perceptual experience, practiced both consciously and by the physical evolution of our brains, that the veracity of perception seems true a priori.
All that objective truth-seeking discourse in ethics has provided is an illusory justification for sanctimony, self-righteousness and oppression. To call one's own ethical beliefs objectively true is to call competing ethical beliefs mistaken, to invoke the normative value of objective truth to override another's will.
If there really were objective ethical truths, this tendency would be benign. The Earth really is round; no amount of propagandizing, negotiation, or coercion of others beliefs will change this fact. But there aren't any objective ethical truths (or at least we've found no way at all to know about them). The illusion of objective truth-seeking discourse has been exclusively used by a self-appointed, self-privileged elite—usually claiming direct or indirect communication with God—to justify what has mostly been at best parasitism and at worst outright exploitation and oppression.
To say that MESR entails that all ethical opinions are "equal" is either irrelevant, trivial or false. To say that they all have equal objective truth irrelevant; it's like saying that turtles and snails have an equal capacity for space travel: No shit, Sherlock. To say that ethical beliefs are all subjective is trivial. To say that all opinions are absolutely equal is false: My ethical opinions are always privileged to me because they are—unlike your opinions—my own (the reverse is, of course, true of you).
MESR does not entail that our subjective opinions are unimportant. It does not entail that we cannot make ethical judgments. It does not entail that we cannot act on our ethical judgments. All that MESR entails is that we can't resolve fundamental ethical disagreements with objective truth-seeking discourse: We must use other modes of discourse (e.g. propaganda, negotiation or coercion) to resolve fundamental ethical disagreements.
Our emotions, our passions, our will—our purely subjective properties—are who we are, as individuals, as members of societies and cultures, as the human species, as sapient beings. As such, they are important, just as important as objective reality. We are not just will-less, judgment-less, emotion-less computers, and very few people want to be such. Our emotions might not be an excuse for irrational thinking, but rational thinking is as well no excuse for denigration and deprecation of the real facts of our subjective properties.
 This is a bit of an overstatement: It's likely that religions and ethical authorities in general serve an important social function, as evidenced by their persistence and prosperity. Any social system based on meta-ethical subjective relativism must identify and fulfill that social function.
 There are some ethical opinions that are intrinsically false: Ethical distinctions made on false-to-fact objective distinctions, such as ethical distinctions made on the basis of the false intellectual inferiority of Black people (or, of course, any ethical belief predicated on one's fantasies about God being real). This case is one where objective truth-seeking discourse is applicable to ethical discourse, although at a derivative—not fundamental—level.