Source : Webster's Revised Unabridged Dictionary (1913)

Indispensableness \In`dis*pen"sa*ble*ness\, n.
   The state or quality of being indispensable, or absolutely
   necessary. --S. Clarke.

Source : WordNet®

indispensableness
     n : the quality possessed by something that you cannot possibly
         do without [syn: {indispensability}, {vitalness}] [ant: {dispensability},
          {dispensability}]