Majority of Med/peds folks per their own professional association reporting actually do take care of both children and adults.
As a pediatrician (I'm a PICU attending), if I were put in charge of GME for a day, I'd immediately change all the FM residencies to Med/peds . The training quality IMO is generally significantly better...if nothing else it's certainly better in regards managing the breadth of pediatric disease. Also think it's a bit much to expect one to stay on top of current trends in 3 specialties like FM tries to.
When I was in academics, I told anyone looking at Med/peds that they needed to feel like they were going to have a hole in their professional soul if they never got to take care of another old person or a baby again to really be the right fit for combined training, that if one of the fields were "just nice to have", then they and their future patients would be better served by them going into just a single field.
There absolutely is a growing need for physicians to manage the growing population of patients aging out of children's hospitals. We as pediatricians have gotten significantly better at keeping these kids alive into adulthood that never used to and it's leading to a population with significant healthcare needs that I doubt are being adequately met. Not saying that's the only use for this training pathway, but it is a great landing spot. Trisomy 21, CF, congenital heard disease, and sickle cell are just a couple of pathologies that come to mind.
While this sounds like a great thing, I think the numbers of places looking to for this sort of thing are probably pretty small. Hospitals small enough to be using FM folks for inpatient management are probably too small to really have any peds beds given the intense regionalization of pediatric medicine. Larger places likely have already divided their patient populations and medical staffs in ways that would probably make the hiring process done in isolation (eg. the adult Hospitalist group needs to hire new staff, they aren't going to inquire if the peds hospital group is in a hiring phase). Maybe after a med/peds person has been hired, they might seek out the opposite group to provide coverage, but I think it's unlikely to be an a priori sort of decision.