It can have a lot of weight. Obviously, different letters will be weighted differently. If the letter is more detailed, is consistent with the medical records, and states a reasonable basis for the conclusions, it will be given more weight. On the other hand, conclusory letters that do not provide details and do not match up with the medical documentation will be given less weight.
I am wondering if you had a chance to look at my post with my NMA from my XO? I am very concerned from what the XO wrote about me. He has only been at my command for 3 months, he never asked me questions about what i have done, how i feel or even who i am that concerns me a lot. Plus I Ghost wrote some of it as i was told to by my Chain of command, and that was put in but then the XO added stuff some of it was true to a point and the rest was a lie. I was hoping you could look at it and let me know what you thought.