This distinction is important because BPD is clearly a disorder of interpersonal relationships and behavior mixed in with a history of trauma and family dysfunction, while true bipolar disorder is a serious biogenic brain disease. BPD, while some of its symptoms do respond quite well to the right medications, should be treated primarily with psychotherapy, while bipolar disorder should be treated primarily with medication.
In fact, as reported in the July issue of the American Journal of Psychiatry (Vol. 173, pp. 688-694), Zanarini and others followed 290 patients with BPD closely over 2 years. They found that "...the symptoms of borderline personality disorder are quite fluid..." This means that they come and go over time. This was particularly true for acute symptoms like self-mutilation. Therefore, people with the disorder may frequently go from 5 symptoms to 4, and suddenly they don't "have" it anymore - unless and until the 5th symptom recurs!
In actual reality, he said redundantly, those people who exhibit three or four of the nine symptoms look a lot more like those folks who have five or more than they do like those folks who have none of them. Now that sounds like a "spectrum" to me.