If you want to do research, you don't even need a MD or a PhD or an MD/PhD for that matter.  You have to be creative, be tenacious and hardworking, and be logical and methodical.  The first two traits are possessed by all scientists whether they are successful or not.  However, to be successful as a scientists your thinking translated into your research must be logical and methodical. What I am talking about is a processes of doing science that is taught.

The training of a scientist is very much like an apprenticeship.  Unlike crafting a fine wrist watch or making the perfect scotch, the apprenticeship you receive from your P.I. (primary investigator) teaches you the fine art of crafting experiments and writing a scientific manuscript. These are very time intensive processes that can only be learned from a patient mentor.  Without this intensive teaching experience, it is very rare for one to learn the skills necessary to produce good research that will be recognized by the scientific community.

One would say that you can always learn this from your one year research elective during your MD clinical fellowship.  However, I would argue that one year (or even 2 years for that matter) is not enough to complete a research project from scratch and see it to completion as a published manuscript.  The process of getting research published is very time consuming and you have to factor in months of rebuttal letters between you and the reviewers, as well as doing experiments to appease the reviewers who will always ask for more and never less. This back-and-forth process is also very much an educational experience and part of life as a scientist, and clinical fellows often find themselves to caught up with clinical duties that they leave it to their sponsoring lab to write the rebuttal letter and complete the requested experiments.

Furthermore, even as a clinician doing research during that fellowship year, you will find yourself having to attend clinics (although less frequently than if you weren't doing your research component of your fellowship) and clinical conferences.  When do you have time to sit down at the bench and not have to worry about patients and clinical obligations?  Will you find time to attend basic science seminars in immunology, cell biology, and biochemistry? The beauty of the PhD is that it is a protected length of time where your only worry is about your experiments.  You will also have time to go to seminars where you can learn how scientists present their research in logical manner and learn new techniques.  Who knows? You may even stumble upon a new technique that may be applicable to your research.

I'll add more thoughts later on why one needs a PhD.