I suppose my opinion is common among traditionally trained dermatologists (IE the overwhelming majority).
I'm also a poster child for training bloat having done an MD/Phd and no longer being in a position where i'm making use of my PhD (like about 70% of MSTP graduates who end up doing either one or the other?). There are all sorts of reasons to do dual this, multi-board this, 3 fellowships, derms, mohs, laser dermpath quadruple boarded superstud...... It all looks good in paper, but at the end of the day, you usually have to reduce the number of hats you wear and be VERY good in your area instead of kinda good at a bunch of things.
So i'm still perplexed at the rationale for med-derm.
1. Do you do med-derm in order to treat systemic diseases with cutaneous manifestations?
Like what? Just how would you see that play out. for me, its simple. I work in a multispecialty tertiary referral center with a 500+ mile service radius. I have ready access to almost any specialist that I can think of.
Moreover, I strongly believe in multidisciplinary teams for complex medical conditions, and it is the model I would personally pursue if i were the patient.
If I have a patient with Crohn's with cutaneous manifestations, I want an Internist managing their general medical conditions, preventative care, and overall state; I can manage their skin, and I would have a Gastroenterologist for their crohn's. Of that team, I wouldn't personally let a Med-Derm trained individual replace anyone except the dermatologist.
Similarly, take a patient with systemic lupus or systemic PAN who has cutaneous disease. They need a Rheumatologist. Similarly, I wouldn't personally let a Med-Derm trained individual take replace anyone except the dermatologist.
In my environment, where I have ready access to highly trained subspecialists, and we are adept at coordination of care, I couldn't imagine someone with this training doing anything but dermatology. Boarding in Medicine ***might*** make you better at managing systemic lupus or crohns than a dermatologist, but i think that it is incredibly unlikely that you will be better than my rheumatologist (and not in a million years will be better than the group of GI docs that I work with). Gonna whip out a colonscope for that Crohns patient, or are you going to manage their "medical-crohns" and send them to an actual GI for their scope?
2. You want to do some general medicine in addition to derm?
Do you really want to manage synthroid and lipitor? Ok. Won't argue with your desire to do that. Can't fathom why you would want to do that. Most dermatologists specifically want to avoid that, and maybe that is a reflection of my "traditional" training.
3. Does med-derm make you a better medical dermatologist? Maybe. Probably impossible to prove. I can assure you that I and my colleagues from residency saw plenty of medical derm and those of us who have chosen to keep doing medical derm are more than willing to go toe-to-toe with you any day of the week on our ability and competence to manage these patients. Perhaps doing some medicine will make you feel more confident, and I can't argue with that, but whether it makes you more competent is an entirely different matter.
Having spent a LOT of extra time on training, I certainly have a bias toward dissuading others from adding additional years to their training. PERHAPS, I am a better doctor because i also went o graduate school. I don't know, really. However, even in the unlikely circumstance that I am, i don't really think it was worth the time I spent.
To be clear, I think that the overall trend in medicine toward training that extends into the 30's and for some 40's is completely unfounded. Even dermatology is managing to find a way to make it longer, and longer and longer. Everyone seems to have to do some lame fellowship before they can match. then if you want to see kids you have to put in a few extra years. and if you want to cut, you need a year (or more) because somehow programs can't find a way to teach that in 3 years.
Rather than advocate for additional yearsof training to be a patho-cardio-laser-botoxirrific-pulmono-neurocutanealdermatoligist, I would advocate to turning dermatology programs into 1+2+1. One year prelim of medicine, peds or TY. 2 years of general dermatology. 1 year of MOHS, Dermpath, Med derm, or Peds derm (if you did peds internship), or advanced clinical derm (or some other garbage can term for people going into community practice who are still going to blend). Certify in derm and your chosen sub-specialty. Done.
) and I think this will make ME a better diagnostician and dermatologist. I know most Dermatologist did not go through this track but I think I would find the extra Medicine year useful. 
