I sort of disagree. I know it depends on the school you're coming from and CV/background, but few people at my institution do away rotations. And, the ones that have told me they didn't think doing away rotations had any real effect on their application cycle. Yes, you can get extra letters of recommendation out of these rotations and you'll get more exposure to dermatology. Yes, I think it can have a substantial effect on networking and getting to know faculty at a specific institution you're interested in attending. But, all in all, these rotations probably have the most effect on your chances at a single institution (the one you rotate at) rather than the 50 or so other programs you're probably applying to. In other words, I don't think 1 vs. 0 away rotations is going to make or break your residency application.
An alternative is to look into 4-week research electives (maybe less popular?) with some sort of defined project (probably clinical research-based). In those cases, you'll probably work 1:1 with a faculty member who could become an advocate for you in the residency committee meetings, even if you're not going to be in clinic to show off your "skills".