[claudiawerner]

The LTV was never intended as a description of what "ought to be" (in fact, Marx cautions against such use), and it was intended to describe how things are. The idea is that people do value things according to their socially necessary labour content, and that this mechanism works "behind the backs" of all members of society. I'm curious in what way it isn't scientifically valid.

The LTV wasn't thrown out because it fails to describe how things are, because soon enough marginalists realize that the same criticisms actually apply to their own theory. It was thrown out (mostly by Samuelson and his pals) of specific issues: the transformation problem, the generalized commodity exploitation theorem, and by extension the Okishio theorem, all of which have been addressed in the Marxian economic literature.

[QuesnayJr]

LTV is clearly normative, and it's why people still try to revive it. The idea that factory owners are exploiting their workers has a certain intuitive appeal as a moral proposition.

The LTV wasn't exactly thrown out -- there are situations where marginalism and the LTV exactly coincide -- but it was superseded because outside a narrow sphere it becomes incoherent. The classic example is technological substitution -- if you have two ways of making something, one that is capital-intensive and one that is labor-intensive, then you will probably pick the one that's cheaper. So you can't calculate the labor content of a good independently of prices.

The specific issues you enumerate are issues within Marxist economics in general. The LTV was superseded well before Samuelson's paper on the transformation problem.