Doubtless I am being a puritan about this because Althusserianism was such a priesthood when I was at college. But I still think that the Althusserians were resolving a perceived problem of economic determinism less well than was originally done by Marx, because he and they never understood Marx's resolution.
Marx's critique of Capital showed that this was an alienation of human powers, subject objectified into substance. The critique of capitalism was to recover man's history-making potential, by revealing the artificiality of the market system, and its openness to human intervention.
A. never understood that side of Marx, relegating it to an early coquettery with Hegelianism. So when he tried to introduce an opening for subjective action into Marx's system, he was alreay misunderstanding it. Marx's system of critical political economy is supposed to capture a form of social organisation that is inherently unfree. Freedom only comes in the act its transformation. Instead A. tries to introduce room for manouevre into the system of capital.
How does he do it? Be introducing many determinations, alongside the 'central' determination of 'the economy'. In this way the determining instance of the economy is made relative, relative to other determinations. The room for political action comes in playing these off against each other, in the interstices of capital, in the Ideological State Apparatuses. This is the theory of the 'long march through the institutions' that led the left into the universities.
But it seems to me that A.'s concept of agency is much poorer than Marx's reworking of Hegel's transformation of 'substance into subject' as the 'expropriation of the expropriators'. A. sees usbjectivity as playing off one inhuman force against another. It is a view of free will that is closest to trading in margins, playing off commodity prices for minute advantage.
A. should have gone not to Engels' later letters to Conrad Schmidt on historical materialism, but to his formulation that 'freedom is the recognition of necessity ... and the leap from necessity'. Seeing the obstacles in your way is the precondition of overcoming them. -- Jim heartfield