The skill to perform good research is not taught in medical school. Not even close.
A masters degree is worth 5 original publications in your chosen field. Write the papers and the MS adds nothing. Which would you trust more as a grant reviewer, someone with an MS in biostatistics or someone who has proven their knowledge of biostatistics in 5 original publications? One has theoretical knowledge, one has proven themselves.
A PhD teaches scientific rigor, technique, and hopefully ethics that will allow you to contribute to the body of literature without dumping garbage onto the scientific community. A few minutes on PubMed and you will see that this does not always work out very well.
I am a funded MD neuroscientist. I have neither an MS nor a PhD. An MD is a useful degree in research because it allows a credible bridge between disease models (vitro/vivo/silico) and human applications. While an MD has the access to patients and clinical questions that make for useful research, without additional training the degree itself does not prepare one for a scientific career.
If you want to do original research as a career with an MD, you need to either get a PhD or be prepared to do a lot of work to learn the tools of the trade. The PhD is a structured track with a well-defined set of requirements, but is still dependent to some degree on your lab and mentors. It is also redundant, painful, and delays your potential earning years even further than the MD alone does. However, getting a reputation for good science with an MD alone requires long hours, fellowships, coursework, and is hugely dependent on your choice of mentors. Pick the wrong mentor and you will founder.
That said, you can be a hobbyist researcher with your MD alone. You can do case-series, retrospective analyses, or even some observational cohort studies without funding. These are all good things, worthy of being proud. But if you want to do RNAi work with a Parkin mouse model with the long term goal of developing a high-throughput drug discovery lab, then you'd better have more than an MD and a smile.