That is the question - and apparently according to this article on salon.com, Flag pins are for losers -- literally.
It essentially boils down to this:
"For these past six years and more, men with those bright little flags apparently riveted to their lapels have fed the voters a daily diet of fear, secrecy, lies and a cruel war with neither point nor end.
No sensible politician would want to march under this tiny, metallic banner. Just look at all the fallen stars who did."