Background: Current categorical classification of personality disorders has been criticized for overlooking the dimensional nature of personality and that it may miss some sub-threshold personality disturbances of clinical significance. We aimed to evaluate the clinical importance of these conditions. For this, we used a simple four-level dimensional categorization based on the severity of personality disturbance. Methods: The sample consisted of 352 patients admitted to mental health services. All underwent diagnostic assessments (SCID-I and SCID-II) and filled in questionnaires concerning their social situation and childhood adversities, and other validated tools, including the Beck Depression Inventory (BDI), Alcohol Use Disorders Identification Test (AUDIT), health-related quality of life (15D), and the five-item Mental Health Index (MHI-5). The patients were categorized into four groups according to the level of personality disturbance: 0 = No personality disturbance, 1 = Personality difficulty (one criterion less than threshold for one or more personality disorders), 2 = Simple personality disorder (one personality disorder), and 3 = Complex/Severe personality disorder (two or more personality disorders or any borderline and antisocial personality disorder). Results: The proportions of the groups were as follows: no personality disturbance 38.4% (n = 135), personality difficulty 14.5% (n = 51), simple personality disorder 19.9% (n = 70), and complex/severe personality disorder 24.4% (n = 86). Patients with no personality disturbance were significantly differentiated (p < 0.05) from the other groups regarding the BDI, 15D, and MHI-5 scores as well as the number of Axis I diagnoses. Patients with complex/severe personality disorders stood out as being worst off. Social dysfunction was related to the severity of the personality disturbance. Patients with a personality difficulty or a simple personality disorder had prominent symptoms and difficulties, but the differences between these groups were mostly non-significant. Conclusions: An elevated severity level of personality disturbance is associated with an increase in psychiatric morbidity and social dysfunction. Diagnostically sub-threshold personality difficulties are of clinical significance and the degree of impairment corresponds to actual personality disorders. Since these two groups did not significantly differ from each other, our findings also highlight the complexity related to the use of diagnostic thresholds for separate personality disorders.