Evenifonly a tinypercentageofneuronsfiredin a givenregionatanyonetime, theenergyburdenofgeneratingspikesovertheentirebrainwouldbeunsustainable.
Here's whereenergyefficiencycomesin.
Lettingjust a smallproportionofcellssignalatanyonetime, knownassparsecoding, usestheleastenergybutcarriesthemostinformationbecausethesmallnumberofsignalshavethousandsofpossiblepathsbywhichtodistributethemselves.
A drawbackofsparsecodingwithin a hugenumberofneuronsisitscost.
Worse, if a bigproportionofcellsneverfire, thentheyaresuperfluous, andevolutionshould'vejettisonedthemlongago.