Is hospitalization for bipolarism necessary, or only in the most worse cases?

Bioolar. Criteria are dangerousness to self or others, poor compliance with treatment, acuity level, inability to care for self, lacking insight, or legally mandated such as court ordered.
Depends on severity. Hospitalization for any disorder including bipolar disorder is determined by the acuteness of the illness and ability or lack of to manage the patient in a lesser intensive level of care.