The individuals I know that do it have a love of both EM and IM. The only complaints I have heard from those same individuals is that you have to be very cautious about ending up as only one or the others. However, we have a guy here who is EM, IM and ID. He works about 1-2 shifts a week in the ED and does an ID clinic once a week.
Others do some as hospitalists and some in the Ed.
You can do *plenty* of research in EM alone. I would say that unless you really love EM and IM, there is no advantage to doing it.