Lacanian theory can be justified simply as an approach that reveals interesting problematics and generates tensions with conventional wisdom. But that's almost certainly unnecessarily modest. Lacan's ontological assumptions are philosophically rigorous, especially when refracted through Zizek's Hegelian rereading of the real as the Night of the World, etc. Far downstream, Lacanian social theory converges with the contemporary canon of rhetorical theory - and especially with the Burkean architectonic - at multiple theoretical points. So there are multiple places where a theorist can post up and point to intuitive structures or conclusions in Lacan.
Critics and particularly orthodox Lacanians sometimes argue, however, that a lot of that work is being done by contemporary Lacanian theorists who take Lacan away from clinical psychoanalysis and bring him into contemporary critical theory. The implied problem is that these gestures go not only beyond Lacan but make moves that are in contradiction with Lacan. One way of articulating this question is as: "how rigorous are people's use of Lacanian architectonic?"
To the extent that these charges are tenable they should be taken seriously. It should never be forgotten that Lacan was first and foremost a clinician - he had patients. Less than rigorous theorists often use of Lacan's mere name as a shiny amulet for whatever theoretically and politically palatable conclusions they're in the mood to assert. Zizek often complains that it's amazing how people find whatever they want in Lacan, no matter how fundamentally their conclusions are in tension with Lacan's basic premises or statements.
But there are other places where Lacan's psychoanalytic work can demonstrably be linked to contemporary Lacanian social theory. The theory of the Big Other, for instance, is a fundamentally a psychoanalytic concept. The Oedipal Complex is resolved non-pathologically to the extent that the toddler gives up trying to be the perfect object of the mother's desire and seeks instead to be the object of the generalized Other's desire.
Starting from the top of social theory and working downwards, the Frankfurt discussions of the Law also map on to the Lacanian Big Other. These are socio-linguistic structures that subjects identify with, that are interpolated, and that are transmitted and reproduced. The similarities are so striking that it's difficult not to treat them as homologies. But the question then becomes: why bother with the theory of the Big Other? Is there any reason to believe it independent of this potential convergence between Lacanian theory and neo-Marxist theory.
After the jump, an explanation of the Big Other in psychoanalytic terms and then the clinical evidence for why Lacan's theory of the drives must be part of any account of interpolation.