The Collaborative International Dictionary of English v.0.48:

Usefulness \Use"ful*ness\, n.
   The quality or state of being useful; utility;
   serviceableness; advantage. --Addison.
   [1913 Webster]

   Syn: Utility; value; profit. See Utility.
        [1913 Webster]

WordNet (r) 3.0 (2006):

usefulness
    n 1: the quality of being of practical use [syn: utility,
         usefulness] [ant: inutility, unusefulness,
         uselessness]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "usefulness":
   account, advantage, advantageousness, advisability, agreeableness,
   applicability, appropriateness, auspiciousness, availability,
   beneficialness, benefit, benevolence, benignity, class, cogency,
   convenience, decency, desert, desirability, effectiveness,
   efficacy, efficiency, excellence, expedience, expediency, fairness,
   favorableness, feasibility, fineness, first-rateness, fitness,
   fittingness, fruitfulness, functionality, gain, goodliness,
   goodness, grace, healthiness, help, helpfulness, kindness, merit,
   niceness, operability, opportuneness, percentage, pleasantness,
   point, politicness, practicability, practical utility,
   practicality, profit, profitability, profitableness, propriety,
   prudence, purpose, purposefulness, quality, relevance,
   rewardingness, rightness, seasonableness, seemliness, service,
   serviceability, skillfulness, soundness, suitability, superiority,
   timeliness, usability, use, utility, utilizability, validity,
   value, virtue, virtuousness, wholeness, wisdom, worth,
   worthwhileness