Brings up something I've never understood. If "To thine own self be true" is such great advice, why did Shakespeare have Polonius, a totally inept idiot, deliver the line?

Was it a cliche even in Shakespeare's time? Was it the Elizabethan equivalent of "Don't take any wooden nickles"? Or did Shakespeare feel sorry for old Polonius and simply throw him a bone - one good memorable line to stand out from the dreck? Or did Shakespeare just not realize that it was good advice? Is it a failed joke that has come to be taken seriously?