The Collaborative International Dictionary of English v.0.48:

Indispensableness \In`dis*pen"sa*ble*ness\, n.
   The state or quality of being indispensable, or absolutely
   necessary. --S. Clarke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indispensableness
    n 1: the quality possessed by something that you cannot possibly
         do without [syn: indispensability, indispensableness,
         vitalness] [ant: dispensability, dispensableness]