I've worked with a fair number of either researcher MDs or MD/PhDs, and myself have only a PhD, so I'll try to give my perspective:
Do You Need One: Probably not, if you're only interested in the scientific questions about what you are doing, and are not particularly concerned with it's direct application to patient care or the clinical setting. Basically, if there is a department outside a medical school that can house your research, you're likely fine.
Why Would Someone Pick One Up?: There are many reasons for a researcher to have a MD or an MD/PhD - they're interested in the clinical aspects of the research science they do, have seen their interests evolve over time toward more research and less patient care, or are in research areas that necessitate having the expertise that comes along with having an MD. There are some medical school programs that are more oriented towards producing new researchers than practicing doctors, and medical schools are massive sources of clinical research in the U.S. It's really not surprising there's a number of professors out there with MDs - my doctoral advisor was one of them.
There's also some...field-specific traditions. For example, there are tons of MDs working in infectious disease research and epidemiology, because it spawns fairly naturally from "Treat this weird infection" to "Why does this weird infection infected X people?" to "How do we stop this weird infection from spreading".