What were the Gadol’s sources? Was it his own call, based on s’vorah and/or his person observation of tzideqiyot? (Either of which is fine by me.) Considering the size of the shaytal industry among even among some convincingly devout females, I wonder if the allure of attractiveness wasn’t always present. If this didn’t merit a p’saq by anyone through the rishonim or from a greater akharon, wouldn’t this alone further reenforce Gadol Rav Moshe’s point? Kol tov.
Shalom Brad, R. Moshe Feinstein (Ev. haEz, I, 58) convincingly proves his opinion from the sources and not just from his own svara (logic). In fact the Talmud (Yoma 47a) cites Kimchit, the mother of the kohanim hagdolim, as being exceptionally modest in that she always covered all of her hair, something which clearly wasn’t done by most other women. This lenient opinion is also already found explicitly in the rishonim (e.g. Rashba, Brachot 24a), and afterwards in detail in the Maharam Alasker (Resp. 35, lived about 500 years ago). You are correct that the allure for a wife to be attractive for her husband, and that shalom-bayit, including the importance of physical attraction in a marriage, is surely a not only legitimate, but important halachic factor. For example, R. Moshe Feinstein himself (Ev. haEz, I, 59) does not allow for a woman to shave her head in most cases, for she will than be unattractive to her husband. Nevertheless, although the Maharam Alasker mentions that the legitimate custom of women purposely leaving out that minimal amount of hair was done in order to be attractive to their husbands, I don’t recall that R. Moshe cites this factor regarding the topic in question (although he does cite the Maharam Alasker). In addition, R. Moshe does add that if their husband agrees, it is worthy for women to be stringent like the Chatam Sofer and Magen Avraham, and cover all of their hair. With Love of Israel, Rav Ari Shvat