Personality disorders are interesting, but since they cannot arise from injury, why would we care if the patient has one?
The interest is in attempting to return a patient to baseline functioning_how they were before they were injured.
If they have a personality disorder of which you are unaware, your efforts to return them to what you consider to be _normal_ will ultimately fail.
Thus, if they are pathologically dependent or pathologically avoidant by virtue of a developmental personality disorder, and you are unaware of this, you can exhaust yourself in an attempt to restore them to a level of functioning which they have never had.
Additionally, if they have even more dysfunctional personality disorders such as paranoid, schizoid or borderline, much of what you believe is arising from injury is actually the way they have (poorly) functioned for much, if not all, of their adult lives.
Think of personality is a foundation upon which our lives are built. A personality disorder can then be considered a weak foundation for the development of adequate social and occupational functioning.
If you have a patient assessed for existence of a personality disorder, you are attempting to establish what was their functional capacity prior to injury and, thus, what is the best you can expect from care associated with injury.
It also alerts the primary treating physician as to what to expect from the patient.