What advantage is there to getting a PhD in the life sciences when you can do research with only an MD? During an interview recently Dr. Fauci, the top doctor in the US, called himself "a scientist, and a physician". It appears that he only has an MD, but is an immunologist and a highly cited researcher. This is not a special case, there are many examples of labs run by a researcher with only an MD. That makes me wonder: why bother getting a PhD in say, immunology, when it appears that an MD is objectively better. With an MD you are qualified to run a lab despite not having the terminal degree of a typical researcher, and you are trained as a physician or whatever specialty so you always have something to fall back on.

To me this seems like the 5+ years spent becoming an expert in your field by doing novel research doesn't really matter as you can get a research job with an MD that doesn't require any of that. So what advantage is there to getting a PhD in life science fields that overlap with medicine?
The trick, actually, is to get that job. Without a PhD or other demonstrated research experience it might be harder most places. Most MD programs are focused more on patient outcomes than on science per se. 

But yes, if you can convince someone to hire you then you can probably learn research methodology along the way. This will include a lot of things, including something of statistics and more on lab process than is probably normal in the MD program itself. Publishing in the scientific literature is also something to be learned along the way and isn't especially typical in the training of an MD. 

The advantage of the PhD on top of an MD is that the focus is precisely on the scientific (rather than clinical) aspects of medicine. You learn to do science under the direction of scientists who also have some expertise in biology and other aspects of the wider medical area. 

Note, of course, that Dr Fauci is 79 years old. He has had a lot of time to learn the craft and has been positioned in places where it is well practiced. 

Note that in some places, an MD is, in fact, a research degree and requires an earlier degree in medicine. But this isn't the case in the US or Canada. See: https://en.wikipedia.org/wiki/Doctor_of_Medicine