Its not that it HAS to be stock solely for the sake of being stock, but at least be tasteful. A lot of us happen to think the car was pretty tasteful to begin with and doesnt need a lot of "fixing" in that regard.
Those tails are just ugly and completely out of place on the back of a Z32. I wouldnt be opposed to aftermarket taillights if they looked good. I just havent seen anyone make any that look good.