If you have done substantial amount of research and published, this letter needs to be included because people will look for it. In fact, not having a letter from such an advisor would be considered a red flag. In many cases, the research advisor frequently ends up being a dermatologist....but not in all cases since people pursue different paths or may have been in a PhD where the advisor may have been in a different field of medicine or in a different scientific field altogether. Clearly these letters should be included. However, on the whole, when applying to programs, your letters are precious and you need to choose the writers well.
As in life everything is a case by case. A research advisor or someone that knows you exceptionally well from another field would count as a great reason to include that letter over a dermatologist. Also, poor enthusiasm for writing you a letter should be a sign that this letter would not be good for you (even if it is from a dermatologist).
I'm not surprised that people found your research letter impressive and you have to know you audience. These are academic dermatologists and many engage in research so they will find that especially impressive. I had all LORs from dermatologists including my research letters. I was told that my LORs were especially impressive and was also told that they really trusted the letter writer because they knew them for X number of years. That's the extra bonus that you get from a dermatologist's letter. They know them. Nothing wrong if you get a strong letter from elsewhere but getting a letter from people known in the field will carry a lot of weight.
I did get an IM letter for my application to prelims and transitionals.