First, I don't think anyone needs both degrees to do good science. I wanted both degrees because I wanted training to be a true physician-scientist. Again, both degrees aren't necessary for this route. But, a PhD provides protected time to really learn how to be a scientist, not just how to do research. I wanted to learn how to approach problems from a basic science as well as clinical perspective. I wanted to learn how to critically read the literature and know what is feasible. I wanted to learn how to design solid experiments that lead to new knowledge.
The great thing about PhD training is that you can make it what you want. You can pick a lab with some cool models that allow you to crank out data and papers. You can pick a lab (PI) that allows you to do safe and risky projects that lead to cool new discoveries. You can learn how to write grants, give effective talks, start collaborations, write papers, etc. You can also pick labs that work closely with clinicians doing translational work.
It is possible to have a meaningful reseach experience as a clinical fellow, but you will never have the protected time to spread your wings, be creative, take chances, and delve into the literature as you do during PhD training.
Having the perspective of a scientist and clinician is invaluable in tackling tough clinical problems in the lab to make discoveries for new diagnostics and therapies. Although we gain so much from animal models, nothing can come about without studying disease in patients.