Bricolage puts in another appearance

"Ideology... is for Marx an imaginary assemblage (bricolage), a pure dream, empty and vain, constituted by the 'day's residues' from the only full and positive reality, that of the concrete history of concrete material individuals materially producing their existence." (Althusser, 160)

What does Althusser mean here? I find it odd that he (or maybe it's more accurate to say Marx) describes ideology as "empty and vain" while he simultaneously conflates it with a concept that, according to Derrida, ought to be in charge of language's construction.* Perhaps Althusser (again, is it more or less accurate to say Marx?) is referring to the fact that ideology is ahistorical, and bricolage itself is an originating force, and that they are both arbitrary in their own constructions. Yet it still strikes me as peculiar that he describes them as "empty" and "null," if only because that use of diction almost implies to me a lack of power, something which I don't think applies to ideology/bricolage at all. I don't think Althusser, by any stretch of the imagination, is saying that ideology lacks power, but I feel (and I admit, it's probably nothing more than a knee-jerk reaction on my part) as if his language unintentionally trivializes the two concepts.

(*Based on this, I think Ioana's question about ideology=the unconscious=language, at least on the grounds of their structure, is an astute observation, and I agree with her that it does appear to be the case.)