Why is the testimony of the ME needed? There is no dispute that Zimmerman shot Trayvon and that Trayvon is dead.
Someone has to prove that Trayvon was dead.
This witness 'might' have been there during the Medical Examination. I don't know that he was in the room at the time, but definitely somewhere near, like the Janitor's office. He has seen a photo of Trayvon dead, so...