So you underexpose because you are afraid of overexposing? Are there no downsides to underexposing every frame? Sounds antithetical to ETTR, not that I ever climbed board that train. Still, it had some purported theoretical basis. I haven't seen much discussion of it in a few years, so maybe it fell out of favor.
From my experience most people who do this, under expose their shots deliberately not because they are afraid of over-exposing the shot across the range of tonal values in the image, but rather to avoid blown highlights specifically. The human eye is much more attuned to seeing great ugly blobs of blown highlight (in the sky for example) and perceiving it as unaesthetic than it is to viewing excessively dark shadows and loss of some shadow detail and perceiving it in the same way. In other words - blown highlights are ugly (if not "fugly"
😎 ) but dark shadows not so much. In fact, photographers very often want deep shadow in some areas of their image as part of the final image's aesthetic. Especially with black and white images. The truth is I want to avoid pretty much any blown highlights but can tolerate all except the most egregiously dark shadows.
I also deliberately under expose images when shooting with digital cameras in conditions which I know will be high contrast and potentially result in blown highlights in part of the image. I should add that I almost always shoot in RAW format too as obviously RAW files help by holding more image data.
Typically, if I am shooting out of doors in bright sunny conditions punctuated by shadowed areas, the shot taken in these conditions with deliberate under exposure (usually two thirds to one full stop under) will come out of camera with less blown highlights but the price I pay is that there will also be some excessively dark shadows as well as some general under exposure. Usually, these RAW files shot in this way still have enough latent detail in the highlights that those details can be fully recovered. Adjusting highlight and shadow tones in post on a computer can usually then fix both problems provided that some latent detail is still available in both and it so happens that this works better if the image is originally under exposed thereby limiting the extent of the blown highlights. What remains can often be fixed in post.
When post processing in Lightroom for example, there are multiple tonal sliders including one that specifically targets highlights and another which specifically targets shadows. These work a treat. I found that this technique of deliberate under exposure was absolutely essential with CCD sensor cameras but still helps with CMOS sensors - even moreso in a way as CMOS sensors have greater tonal latitude and hence better ability to pull detail out. (I accept however that Monochrome sensors are less able tolerate this kind of manipulation in post due to there not being multiple color channels but for this reason under exposure of the image is perhaps even more advisable - i.e. there is less latitude to fix it later.)
You are quite right in saying that this practice contradicts the "expose to the right "rule" but I have always thought that ETTR was a concept which makes sense if you are shooting in studio conditions where lighting is totally under the photographer's control and he/she is seeking absolute image quality, but does not make sense in out of doors lighting conditions where lighting is outside of the photographer's control and you are forced to take whatever nature gives you. If this is likely to result in blown highlights (which these shooting conditions often can do) then it makes eminent sense to mitigate this by under exposing deliberately and accepting that you are theoretically losing some other detail.
