I've known a lot of officers who wore their expert marksman ribbons (including me).
Most officers I've been around don't wear one. I've always be taught that officers are expected to be expert shots, therefore wearing the medal is redundant. I always shot expert, with both the 9mm and whatever M-16 variant I shot, and have never worn mine.
Probably because the majority of officers don't earn one. If everyone's an expert, the standards are too low.