WHEN the economy was sound and medical care more affordable, comprehensive national health insurance was a top priority measure on the liberal political agenda.  The straitened circumstances and escalated medical costs of recent years, however, have compelled analysts and officials to lower their sights. Seeking something short of national health insurance that would nevertheless insure people against the crippling costs of severe accidents and illnesses and long-term hospitalization, they have hit upon the simple idea of “catastrophic health insurance.” Such coverage, it is believed, would provide Americans appropriate care for unavoidable high-cost illness at a fraction of the cost of any comprehensive care program. But is this the case?