I'm doing a one year post-doc in my old grad lab, I graduated MD PhD and matching this year (my wife is a year behind me in medical school). A friend of mine is an MD/PhD who couldn't decide between IM and Psych, and decided to do a one year HH fellowship at the NIH in proteomics. A third person I knew left our MD/PhD program for reasons, and did the 2 year version HH fellowship at the NIH in software programming for imaging.
As far as improving your CV, its field dependent. Radiology, Gen Surg, Ortho surg, not so much. Rad onc., Psych, IM, Neuro, Path, Neurosurg loves the research. But FYI, YOU should LOVE research. I've seen several people flame out in MD/PhD programs, to switch to pure MD or pure PhD. If you're applying to research heavy programs for residency, it will help, but if you're just looking to generally buff your CV, DON'T DO IT, YOU WILL BE MISERABLE. Do a case report or some other quick easy publishable project. Plus, programs that don't have significant research resources will wonder why you are applying to their program, if you are so into research.
Basically, if you have a 2 year fellowship, you will probably be published, have a good project you completed. You also most likely have the initiative and patience to continue research as a research fellow after/during residency, and bring in research funding as junior faculty, which improves the department's prestige and bottomline. But if you just want to be a clinician, this research is kind of like an expensive pen and pencil set, a nice thought, but kind of pointless.
Hope that helps.