I did use my carry letter, It was "required" living in Illinois, it was necessary to present it to the attendant at the DMV in order to have my gender marker changed.
it had to be the "original" letter, and they "KEPT" it. providing me with a "copy" of it.
so now i have a drivers licence with the gender marker "female", and a copy of the original carry letter.
so I can say without hesitation, the carry letter IS required, at least thus far. now i supose it's just a formality to have it.