Mental illness is not an ‘illness like any other.’ Mental illnesses are contextual in nature. In some contexts, being obsessive-compulsive is a liability, and thus an illness, and in other contexts, being obsessive-compulsive is an asset, and thus not a mental illness.
And the same is true, to varying degrees, of all other mental illnesses.
By contrast, physical illnesses are not contextual in nature.
If somebody has asthma or lupus or cancer, that is not asset in some contexts and a liability in others: it is simply a liability.
Also, mental illnesses serve a defensive function.
Even though, in at least some contexts, OCD is a liability, it serves a protective function: it insulates the subject in a psychological ‘safe-space’: his obsessions and compulsions encircle that safe-space and, although dysfunctional in some respects, do provide him some measure of safety from a toxic and threatening environment.
And the same is true of psychosis: the psychotic did not become psychotic for no reason. He was in a situation in which he was being threatened and pumped full of negative validations and detriments to his psychological well-being and sense of self-worth; and on that basis he decided, with some degree of rationality, to distance himself from that destructive negativity by withdrawing into a world of psychotic delusion.
So even though psychosis is an illness and is gravely dysfunctional, at least in some respects, it does serve at least some protective function.
Also, to develop a point hinted at earlier, some mental illnesses, and possibly all, involve misdeployed strengths of intellect and character. OCD involves misdeployed intelligence and focus; schizophrenia involves misdeployed creativity and emotional sensitivity.
So mental illnesses, though in many respects gravely debilitating, seem to involve heightened, albeit misdeployed, abilities.