Being a current Harley rider that subscribes to the ATGATT principle I can at least weigh in with my personal experience. I have an Ultra Classic (the huge full dresser with fairing and radio) that I got following riding a Triumph Thruxton. I have always rode with a modular or full-face helmet, riding jacket/pants/boots/gloves. The first ride of any distance I did on the Harley I found myself sweating profusely and burning up inside my helmet thanks to that huge fairing. I went out and bought a half-helmet to help with airflow and buffeting I was experiencing with my modular. I rode with this for about six months before talking with a friend who mentioned a simple thought about how he could live without an arm or a leg, but not a head. While I do not wish to live without any of the above, I could not argue with that point and went back to wearing the modular.
My personal experience is that the airflow behind the fairing is atrocious (which can be corrected) and can lead to the decision to wear less protective gear. I think the vast majority though do not wear significant ATGATT because of the "Harley biker look" rather than any consideration or intentional disregard for personal safety. There is also a certain group that has an opinion that no amount of gear truly makes a difference so they'll don none at all. I think a good cure for that could be reading the face plant forum.