Flying instruction theory, particularly ab initio instruction, should concentrate on items which have a practical significance.
The test on whether a particular item of theory should be taught ought be " is there a potential circumstance where the student can apply the bit of knowledge I am about to teach "
So going back to the OP's question, knowing a mag compass has errors and learning how to compensate for those errors has obvious practical significance.
Teaching the design of the compass and how/ why the design produces those errors has zero practical use so IMO it is waste of time. That training/ study time could be used to teach things that are actually useful.
I think there is far too much emphasis on theoretical subject which have absolutely no use in the real world of flying an airplane. The UK is IMO particularly bad in this respect
Here is a heretical thought. Given the, on average, once a month carb ice induced engine failure and resultant forced approach maybe ground school less time should be spent on the theory of compass errors and more time on
recognizing the symptoms of carb icing and knowing those conditions when it is most probable
Hat, Coat, Door ....