I really can't see, why true colour icons would be so useful... Other than there wouldn't be need for any 256col=>WhateverColordepthUserIsUsing conversion...
(Of course, they'd need to be dithered, should anyone use <=16bit modes)
256 colour icons allow use of 256 independent colours within icons. Theoretically, you could waste them on 16*16 icon (16*16=256) but this is hardly ever the case...
