Here is the simplified logic behind why I find Marx's Law of Value Compelling:
(i) Value and price are not the same thing.
(ii) A bubble bursts in the economy when previously inflated prices are corrected. (Note that "correction" is not my own term, but a term frequently used to describe such phenomena in economics.)
(iii) The only way to make sense of this is that prices originally (prior to the bubble bursting) deviated from values too much.
(iv) If it is the case that prices can deviate too much from values while prices are derived from marginal utility, value cannot be subjective. There must be an objective substance of value around which prices can deviate (to an extent).
(v) Therefore, STV is invalid as a theory of value.
(vi) Having accepted this logic, it follows that we require an objective theory of value as opposed to a subjective theory of value. Now the question becomes: What should this objective theory of value be?
(vii) An objective theory of value must express value as being comprised of some definable substance.
(viii) Given that we have established value as something objective rather than subjective, it must be possible for commodities to be exchanged in such a way that there is equal Value on both sides of an exchange.
(ix) In order for things to have equal value, the substance of value must be some characteristic that all commodities share but also separates them from non-commodities.
(x) The only such characteristic is that they can all be produced by simple/"unskilled" human labor.
(xi) Therefore, expressing the value of a commodity must be done in units of simple/"unskilled" human labor.