I am currently a resident at a university program that has a Med/Derm program and I'm friends with a few of the Med/Derm residents.
The goals of the Med/Derm program are to train academic dermatologists that are firmly grounded in the principles of internal medicine. The idea, from my understanding, is to ideally manage patients such as those with cutaneous manifestations of lupus, Hepatitis C, IBD, etc.
However, I honestly don't think those trained in Med/Derm are any better than those trained in only Derm. I think it's moreso the differences in the individual clinicians in the end. Obviously there's very intelligent Dermatologists who never double-boarded in Internal Medicine and Dermatology.
Additionally, I really don't think there are hospitals that are looking exclusively for someone who is Med/Derm certified. The graduates I know of the Med/Derm program here all went on to practice exclusively in Derm practices after finishing, and my guess is that 10 years down the line, most of the Internal Medicine they learned in residency will pour out of their brains unless they have some level of continuous exposure to it.
Anyways, Med/Derm definitely isn't a "backdoor" into Dermatology. The residents are from prominent medical schools, AOA, I'm sure they had stellar board scores, etc.