I did a conversion with Nikon Capture NX and noted the same behavior as with ACR. I also did a conversion with RawMagick, which is a well regarded converter that uses floating point math rather than integers.
Here is the shadow portion of the histogram:
and towards the midportion:
The missing spaces on the left are present, but irregularly spaced. It could be that they were filled in in the linear file, but the holes appeared during the gamma correction as you predict.
With Histogrammar we see all types of things that were not apparent in low resolution histograms.
I think I have just found the reason for the discrepancy (peaks in DCRAW, no peaks rest of RAW developers). The key is colour profile conversion: -o 0 option in DCRAW develops with no colour profile conversion (as far as I know: just black offset substracting, WB, 0..4095 range adjust, and Bayer demosaicing). But commercial developers additionally always convert to an output colour profile (sRGB, AdobeRGB...).
We previously saw the peaks in DCRAW's developing with no colour conversion.
This is what we get in DCRAW linear with sRGB colour profile conversion (-o 1):
No peaks (and Zoom=1), the colour profile conversion reallocates all levels in a very uniform way, making the original captured and interpolated values absolutely indistinguishable (this word exists?). By applying gamma over this last histogram we would get a similar result as in ACR and Capture NX: locally-equally spaced matching RGB levels of similar amplitude.
BTW I wanted to ask you something that disturbs me a bit: I almost have no idea about colour profiles, but as far as I know each colour profile (sRGB, AdobeRGB...) has its own standard gamma curve. On the other hand DCRAW allows to develop a RAW file and convert it to a given colour profile, but IN LINEAR MODE, i.e. not applying the gamma correction yet.
Is this conceptually correct? can the gamma curve be applied after a linear colour profile conversion as a separate stage of the conversion process?
David Coffin (author of DCRAW) told me he puts in the output TIFF file converted to a colour profile the needed information so that for instance PS recognizes it as a linear (still gamma=1.0) image. And it seems so: when you load this tiff files into PS (indicating it is gamma=1.0 in the Edit->Colour adjust->RGB custom menu), and then you convert to a colour profile (which can even be the same that we previously set in DCRAW), PS clearly expands the histogram by a gamma curve.
What I wonder is if all this process is correct from the perspective of getting a correctly developed image or we are just getting "numbers" that in some way look like what our photograph should have been.