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.
Carry letters are always a good idea when traveling, such as on airliners, or when your ID does not readily match your current appearance.
I think though, once you have up-to-date ID, like a drivers license or passport, you are unlikely to be hassled.