The fender protects the light from rainfall. Right?
It also protects it from prying eyes.
Without the fender in place, the tail light would barely be above the curve of the rear tire anyway.
People's seatpost fixation makes many tail light placements nearly useless. If you just want to create a personal delusion of safety and use up a lot of batteries, go ahead. If you want to be seen, get those lights up and out.