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The birthers argument isn't really about birth certificates. It's about whether or not a non-white can legitimately be President of the United States.
Of course, these days you can't just come out and say "No Negro can ever legitimately be President of the USA", because you'd be dismissed as a racist.
So instead, you pretend it's about Obama's birth certificate, or alleged lack thereof. That way you can question Obama's legitimacy without having to look like a racist kook.