Essentially Bannister is concerned about the fact that any given pair of people with a diagnosis of Schizophrenia can have entirely different behavioural presentations. On the face of it this looks very problematic; Schizophrenia is behaviourally defined, so it seems a little counter-intuitive that the definition can-in theory- capture radically different sorts of behaviour without any overlap among them. Bannister's "disjunctive" point therefore seems to land a blow on those who do research into Schizophrenia, and it provides critics with a nifty sounding slogan for their claims ("Schizophrenia is a disjunctive concept, so there!"), but to what extent should the "disjunctive" argument actually be a worry?
Although Schizophrenia is defined behaviourally, it has been a key assumption throughout the term's history that there is something behind that behaviour that requires explaining. Psychoanalytic theories have put more emphasis on a loss of ego boundaries and self-integration while biological theories traditionally focus on the action of neurotransmitters. Modern trauma theories put more stock in the notion that dissociation may play a role. What all these explanatory ideas have in common is that they attempt to explain the diverse range of behaviours that lead to a diagnosis.
Without any prejudice as to aetiology or mechanism therefore, we can say that the behaviour of a person with a diagnosis of Schizophrenia is not meant to be the main fact about them that determines the presence of Schizophrenia. Instead, the diagnosis is conferred when the behaviour of the patient gives the clinician some reason to hypothesise that the underlying process (aberrant dopamine signalling; disintegration of the self/other boundary) is taking place in the mind or body of the person being assessed.
We can of course argue that the diverse presentations of Schizophrenia are not in fact caused by a single underlying process, but that is an empirical matter. If there is a single underlying process (or family of interconnected underlying processes), then the fact of Schizophrenia's being behaviourally "disjunctive" is no more interesting than the fact the fact that the same virus can lead to both diahorrea and vomiting. If there is no underlying process (or family of interconnected underlying processes) then that speaks against a single "Schizophrenia" whether or not it is "disjunctive" in Bannister's sense.
Now, just because the "disjunctive" argument may be a red herring, it does not follow that the concept of Schizophrenia can have a free pass in the clinical and scientific lexicon. There are many reasons to be dissatisfied with Schizophrenia-talk, both from a scientific perspective and from a political/social care perspective. My point is not to defend all the ideas associated with one limited reading of "Schizophrenia", nor is it to seek necessarily to preserve its use as a term. Rather it seems we should focus our attention on thinking about what sort of thing or things we really believe "Schizophrenia" refers to.
Many researchers are well aware of the contested nature of Schizophrenia and their work is about understanding how viable a category it is and what is actually going on with people who get the diagnosis. They know that the Schizophrenia of the 1960s is radically different to the Schizophrenia of the 1980s, which has in turn evolved between then and the present day. The fact that a research programme is oriented around the broad family of issues that goes under this name should not be taken the a sign of institutional myopia that many believe it to be.