With the don't-care allowable, the "load" nets can be tied directly to the instruction decoder (i.e. LDA = BIT0, LDX = BIT1) instead of needing intervening logi

With the don't-care allowable, the "load" nets can be tied directly to the instruction decoder (i.e. LDA = BIT0, LDX = BIT1) instead of needing intervening logic (i.e. LDA = BIT0 & !BIT1, LDX = BIT1 & !BIT0). If you can make the opcode illegal, you can save two gates (which matter for cost, yield, power and timing).