Okay, from a designer's perspective, there are usually 2 reasons we chop of the head, and they're not about reader fantasy:
1st is regarding the model's privacy and that they might not like how their image is being used. If the cover is for an erotica, the model may object to being identified. There have been cases of models complaining about this in the past. Showing just a muscular body with a hint of a jawline keeps the model from being identified. For the designers POV, we don't always know how the cover will be used. Especially with premade covers. Authors also change their mind later and buy a cover for a romance then use it on an erotica instead (or the nature of their book changes as they right it). It's not just for erotica, though. Suppose you have a high heat MM bear shifter romance and the model doesn't want to be connected to a MM or shifter book? Many of the photos sites warn against depicting an identifiable model in a way that may cause embarrassment. That's where the genre comes in. It's much more likely that the model would be embarrassed by being associated with a gay shifter romance or a dark BDSM versus being the face of a detective or adventure story, etc.
I find your post extremely interesting. At least for me: I'm a just arrived here, a newcomer. But it gives us illustrators some hope in the industry. Nothing easier than to generate from scratch any human or creature, with the exact features, personality, vibe and even face expression/feel of the moment, without restrictions... I can see how to be very accurate in this regard is extremely difficult if dealing with (even a huge catalog...been there when told by a boss, me working as a graphic designer, being told to absolutely depend only on stock photos. What a pain it was. I can only show my deepest respect for those producing those wonderful covers by doing so) stock photos. Tends to be extremely difficult, even hours of searching in several catalogs, to find that EXACT image (never happened) you are after. Only matched by looking for a specific font...
But an illustration only needs to ask for permision to own's imagination, happily. (of course, full training in anatomy drawing and etc is a must. But if speaking of a relatively high level, usually not the average teenager (usually! there are genius...). The pity is people tend to go to those, thinking is going to be cheaper (surprisingly, it is often not the case)). Of course, the hugest problem is the pricing in illustration (my take is it we all should lower certain bars there, but I'm alone in that one...). There's a ton of opportunities for illustration in book covers, IMO. But might never happen massively, as it takes time produce one, and people want their deserved payment, of course...I still think it's possible a different take at it, though.
And this not going against a particular trend's kind of established depiction. I believe one can stick to that, whether doing it as design, illustration or 3D (but if illustration takes more time than design, 3D takes way more, even. Done at the needed quality level, that is). I personally would choose to have a bit of both, as an artist: I understand the business rules... I know what is it to eat your artwork for not going for what people is hunting for. In the other side, I believe I'd allow my self some premades in entirely my strongest, personal style. IMO, that's healthy for the artist: IE, I wouldn't worry if a 10% of the portfolio wont sell, if it is showing what I can really do in custom work, for the "brave enough " clients.
I'm not making a point that this is right or wrong (or that the model should have thought about that before posing, because I always feel that "should have" is one of the most obnoxiously argumentative and least helpful phrases in the whole language). Generally speaking, the higher the heat level of the cover and its expected book, the more a designer should consider hiding the model's face. IMO at least.
I can completely understand the models' concerns... Most model licenses are complex, too. Well like most licenses are, in general. I wouldn't criticize that when I am myself extremely cautious with all related to privacy. It's their job, I know, but still, is completely human that they might have concerns, varying those from person to person.
The second issue is also with premades or sometimes custom. There are only so many stock photos out there (and there are many more photos of women then men), and writers are going to want their cover models to at least generally resemble their characters. Suppose their lead is of the tall, dark, and handsome variety but the stock photo that best fits the cover feature a tall and handsome model with blond dreads? Instead of trying to edit the hair which will raise the cost, it's much cheaper and faster to simply hide the incorrect feature out of sight.
Yeah, another perfect case for (cheap yet good) illustration... :D . In any case, it can't help with the mass production in any way, can't compete with graphic design/stock photography. The indie publishing scene looks to me as an incredible machine producing books... I don't think we have enough good(and inexpensive) illustrators in the entire world to keep up with that crazy pace and massive production, lol.
Not sure if has been mentioned, though, please excuse me if it did (some parts are old posts, can't remember), but another reason why (and this would apply to photos, 3D or illustration) to "chop the heads" (btw, a general no-no in general art composition across several fields...but all is context, anyway) , it could be that... well, in some cases, the author has such a particular, specific view of the main character, that probably no illustration, no photo, no 3D is going to totally, absolutely nail what is there in the author's imagination, satisfy that depiction. I believe that type of case could be like a 5% of the cases, but still, for those chopping the head might at least not make the author see a face which feels strange, is not the very exact person in mind, even having all features provided till every depth. From what I read, most people would be happy with a 98% of approach to the own concept, or even a 70%... And in a large bunch of cases (of stuff, posts, cover petitions, etc, that I've read till now) , just that the character matches the overall global genre specs and the story...
