Can an EM certified doc practice as a family practice doc if s/he wants?
EM trained docs can work in more than emergency departments. They can do Urgent Care work, Occupational medicine, and could probably be approved to staff most any walk-in clinic. There are many such clinics in rural areas where it can be very difficult to get anyone to staff them, and the organizations that run these clinics would probably not have a problem with an EM-trained doc working there.
At one walk-in clinic near where I work, a former general surgeon from our hospital is on the staff. And at our hospital, we have some internal medicine and FP-trained physicians who have never done anything BUT emergency medicine work. Internists and Family Practitioners who work in ERs usually have taken all the requisite "merit badges" (ATLS, ACLS, PALS, APLS), read Tintinalli or Rosen, and gotten whatever additional training (e.g. CME courses) they need to get the job done. So there's no reason a physican from a close specialty couldn't "cross-train" and do a competent job.
With that said, although many internists and FPs can competently staff emergency departments, the real questions are
1. Is this best for the patients?
and
2. Is it best for the specialty?
The answer to the first question probably varies from physician to physician. There are certainly some FP-trained docs doing a very good job in ERs, and there are probably some incompetent emergency medicine residency trained docs out there as well.
In the long run, it is probably the emergency medicine residency trained docs who are best suited to handle critically injured patients, since that is what they are specifically trained to do.
As for the second question, it is certainly in the best interests of the specialty to have emergency medicine residency trained docs doing all the staffing. The whole point behind having a specialty is the assumption that a given group of patients, whether it be kids, adults, the mentally ill, or the critically ill/injured, require medical practitioners who are specially trained and have a specific set of skills best suited to deal with that group's specific needs.
If that is not the case, then one cannot argue against having FPs, etc. staffing emergency departments, or against having residents from other specialties moonlighting in ERs. Such habits perpetuate the notion that "anyone can staff an ER," a belief that the specialty of emergency medicine is trying to combat.