I would think that having 1 glowing LOR that mentions "transitional year" would be just fine for optho or other "advanced" programs, as long as you have some sort of ophtho letter(s) also.
I wouldn't send this letter to medicine categorical programs that are your "backup", unless you have no other choice. The programs are going to know that you are using their medicine categorical program as a "backup" and they won't like this. This letter might be OK (probably would be) for medicine prelim programs b/c they know they are only going to have you for 1 year anyway, and might not be shocked that they are competing with transitional year programs to get you.
