Gavin, do you have a source for them being added for aesthetic reasons? I would've thought it's because air brakes are stronger than vacuum, and the trains 7 and 8 could handle almost necessitated the extra braking force compared to 3 or 4.
Also, if aesthetics were the reason, then it seems awful strange that 8 was regularly outfitted with a box headlight and faux diamond stack, even with air brakes.