Do people who criticize Marx's LTV even understand it to begin with?
So many people are convinced that Marx's account of LTV as socially necessary labor-time has been "disproven" by the concept of marginal utility. I don't think such people understand Marx's distinction between use-value and exchange-value. To begin with, use-value refers to what others call utility, which is to say that in order for something to possess exchange-value, it must already possess some use-value to someone. In other words, Marx is already careful enough to point out that value is intrinsically tied to utility. The marginalists have a shallow view of the matter. Whereas Marx understands value as both "objective" exchange-value) and "subjective" (use-value), the neoclassical school understand value as strictly subjective. At least that's my understanding of things. They conflate the two aspects of what constitutes "value."
I also don't think the concept of diminishing returns as it applies to the idea of marginal utility is significant. The only reason one can evaluate an extra unit of X item as possessing less value than a lesser number of that same item is if it's capable of being produced at a faster rate with the implementation of more advanced technology, which presupposes a reduction in labor-time. For Marx, a commodity loses its value because it is produced in less time than previously. If there are more of said commodity exchanged in the market as a result of this increase in production, then the subjective evaluation, which Marx acknowledges, of said commodity is naturally going to accompany such an increase.
Are opponents of Marx simply not reading Marx? What's going on? The cynic in me says that idle intellectuals in the 20th century were offended by the notion that they lived off the products of another person's labor, so they effectively rationalized their parasitism by reusing concepts that were already popular in Marx's time.