I really don't have enough knowledge to answer your question, but I did ask A Forensic psychiatrist this question during a grand rounds.
He said there's such a strong demand for FPs that if you just want to be a successfully practicing FP you don't need to worry. However if you want to become eminent in the field, put that feather in the cap, and make history, by all means pursue one.
Another FP told me at a grand rounds that sometimes combined JD-FPs strongly overdo 1 vs the other, and if you're working with a good lawyer, they're going to fill in the gap for you anyway, so don't worry.
Aside from my age (33) and my desire to just start making money, & stop cramming for exams, I wouldn't want an FP (edit: meant to say JD) because not working in clinical medicine for some time makes you forget clinical medicine. If you got a JD, I'd think that you'd forget most of your clinical skills, to the point where you'd have a tough transition back to clinical work if you wanted to do a combined clinical practice in addition to working as a FP. Heck, when I take a week off, I start to forget stuff that I think is common knowledge on the hospital floor.