There is a bottleneck in the number of MD positions. Without the MD title, your typical life science researcher cannot easily carry out medical research. Medical education and training has a scale problem.
To conduct good clinical research, I think you need to understand the constraints of clinical practice. It's not only a matter of holding the title, but more of practical experience.
You do need someone with the clinical experience to understand the questions that need asking and to confirm the trends and suspicions seen in clinical practice, but clinical academics are expensive, and also terrible at stats and good research practice. I think the right mix of both is important.