Dear Giuseppe, in my opinion, the practice of introspection is useful for who want to investigate its own conscious (self-conscious) contents. The Freudian Psychoanalysis focuses on the unconscious contents of mind, which are distorted by the pre-conscious apparatus before they can be consciously accessed. So, introspection practices may not be useful for psychoanalytical purposes. Perhaps, the absence of such theme in the Psychoanalytical mainstream may be justified in such way...

In psychoanalysis, introspection is intimately connected with counter-transference (the analyst needs analyze himself to become aware of it), but I'd say most Freudian psychoanalytic theory was developed through introspection. The famous Oedipal theory, for instance, resulted from Freud's self-analysis.