There are at least two major disadvantages to the method of shooting at ISO 100 and allow underexposure 1) You can't even see the pictures on the LCD, you can't review focus or show it to anyone 2) you have to adjust exposure when importing into Lightroom or any other RAW converter before you can review the pictures for selection or rejection.
Hans,
First let me congratulate you on your spectacular landscape images on your linked site.
Now to business.

I've already agreed there is a disadvantage in not being able to share the images with anyone on the camera's LCD screen. However, if one was attempting to capture the moment, the ability to review the image for focus and other issues is not likely to serve much purpose. Would Cartier-Bresson have found the ability to review his shots useful? Refer attached image.

The extra work of adjusting exposure in the RAW converter before determining whether or not to delete an image is trivial for me. By far the greater proportion of my rejects over the years have been those taken in Aperture Priority mode when the automatic selection of shutter speed has been too slow to freeze movement or too slow to avoid blown highlights in parts of the scene that I considered important.
It is this very preponderance of such rejects that has led me to this approach of using the camera as though it were ISO-less. Nevertheless, I accept there are small, incremental losses in image quality, compared with using a 'correct' ISO in relation to an ETTR, which might be noticeable if the underexposure is taken to extremes.
For me it's really a trade-off between the two concerns of (1) getting the shot with a confidence that there will be nothing seriously wrong that will cause it to be a reject, and (2) missing the moment entirely as a result of spending too much time making camera adjustments to ensure maximum image quality.
Your approach of using auto-ISO should be better when using extreme underexposure. I'll give it a try. My main concern is that there appears to be no indication in the D7100 viewfinder of underexposure when a minus value of EC is enabled.
Check the examples of my ISO 6400 test I have just posted. Try it with your D7100 and it will likely be much worse.
Now, Hans, you are getting a bit tricky.

I mentioned specifically that Nikon is going backwards with regard to the ISO-less nature of their most recent models, including the D800 and the D7100. A consequence of this is that one should
try to avoid using ISO 100 when underexposing. You've not only used ISO 100, but given 6 stops of underexposure as well.This is what I wrote earlier, in reply to BJL:
If we compare the DXO measurements for the D7100, we find that the ISO-less nature is not quite as good as that of the D7000 and is best within a narrower range of ISOs, specifically from ISO 200 to ISO 1600, which represents the 3 stops underexposure that I used in my above shot of the mule.
Just to confirm that I am correct in this view that there should be no significant problem in underexposing by 3 stops at ISO 200 with the D7100 (apart from possible banding), I took the following shots just after sunrise this morning, Australian time.
Perhaps I should not have labelled these shots, and should have asked you to guess which was taken at ISO 200. I simply cannot discern any differences at 100% view that would indicate that one of the shots was an underexposure in relation to its ISO setting. Can you?
I should also point out that I have made no adjustments to either of these images apart from raising the exposure of the underexposed shot to +3 in ACR, and apart from letting the default sharpening of 25 apply to both images. The conversions are from the settings that appear in the attached screen capture of the ACR window, with no further adjustment in Photoshop.