The emulator has to handle and the testing infrastructure has to have tests for each subregister and how it affects its parent register. You have to prevent and

The emulator has to handle and the testing infrastructure has to have tests for each subregister and how it affects its parent register. You have to prevent and detect design-level oversights such as "AH is not the LSB of AX/EAX/RAX and my design only accounts for LS-whatever subregisters". Your point that they can't be distinct holds, but but if they were distinct they would have less impact on the engineering than they will have as subregisters.