Why do we hide the real value of our PhD?

I've been following some threads on the discussion boards over the past few months where there is a battle raging about whether to leave your PhD off your resume in some circumstances in order to address the “Overqualified” part of the “overqualified and under-experienced” curse.  This is all based on anecdotal stories about postdocs who have been passed over for jobs and have either concluded themselves, or been told , that the reason is that their PhD “over-qualifies” them for the position.