You have to keep your focus in a context. I don't need Einstein's Theory of General Relativity to perform all sorts of tasks. You do not have to 'check' the theory of gravity to conclude that you need to be careful whete you set that piece of rare china. Theory at this level has NO (direct) link to most human practice.
Suppose Marx's critique, as construed (say) by Postone is correct.
Does anyone need to consult that theory to work out a particular campaign in a mass struggle? NO.
Does one have to consult that Theory to be opposed to capitalism? Obviously No. There is no link between the theory and anti-capitalist practice.
So what you say here is empty at most levels of theory. Most of those who turn out for local rallies or demos don't first consult a theory, then say, according to this theory I should go to the demo today. In fact, try to drag theory in here and you will end in a hopelesds muddle.
We are talking about both theory and practice here at a high level of abstraction, and they do not connect. There is a good deal of thought, a good deal of careful abstraction, involved in identifying the relation of theory and practice at any given context. Blunt statements like yours don't help.
Carrol