I would venture that even grunts are capable of adding and subtracting.

Just kidding, but how would you implement declination adjustment in a rotating card compass used in the sighting mode? With the Dakar it looks like you rotate the lubber line on the face plate, but this would introduce a parallax error when not looking down from directly above.

One more argument against declination adjustment - if you make a conversion error while using a compass without adjustment, one bearing will be wrong. If you make a mistake setting the declination on your adjustable compass, all your bearings will be wrong.

In North to the Pole, the author writes how they travelled South instead of North for an entire day because he set the declination wrong.