Functionalist explanations are objectionable in that they embody what might be called a "systems teleology": a supposed functional requirement is held to call into existence the institutional complex (a state, ideological apparatus, or whatever) which satisfies the requirement. In the case of Althusser, the decentring of the individual subject is achieved at the apparent cost of a reemergence of conscious agency, or its analogue, at the level of the social systems itself: 'there is indeed a functionalism of this sort in Althusser. However, a good deal in the "reproduction" approach to the state and ideology can, in my view, be sustained without it. It seems to me to be perfectly legitimate to ask, either at the level of society as such, or at the level of specific sets of social or economic relations, "what are their conditions of possibility".' (p. 222) Benton 1984
'Even if one could accept the functional assumption that the existence of functional requirement calls into existence its satisfaction, this still falls short of an explanation of how it is that "answers" to functional needs are distributed among institutional complexes in the way that in fact are.' i.e. functionalism can not necessarily pin point why one social function, e.g. that of integration should fall upon the state rather than any other social body.
Under functionalism, social change is impossible to identify, as it conflates 'questions of genesis with questions of system integration' - leading to the idea that social realities are eternally self-producing. Furthermore, the concept of ideological interpellation leaves no room for oppositional forms of subjectivity, and conflates ideology with ruling ideology.
However Benton suggests that a modified Althusseriansim, that adopts the later perspective of class struggle as a motor, is appealing because it does not conflate questions about effects of institutions with questions concerning their genesis:
'The achievement of political and social institutional forms through which the reproductive requirements of capitalist relations can be met, is not a magical result of the teleology of the system, nor is it a purely contingent and fortuitous happening. It is, instead, the uncertain, uneven, qualified and contradictory outcome of struggles between opposed social and political forces. In these struggles neither side is guranteed success, and neither side has a God given perception of its interests and how to secure them. That however either side even partially achieves its objectives is a testimony to the real cognitive content of the ideologies through which these social conflicts are fought. However partial and distorting these ideologies are, they must necessarily have a degree of correspondence to the practical requirements of struggle.'
In this E.P. Thompson is certainly vindicated against both Althusser and Hindess and Hirst.(p. 225)