Do ACS letters count for anything or are they worthless?
In what way? A letter from Anthony Fauci is worthless if he says you're the worst medical student to ever darken his office door. Likewise, an honest, strong letter from an attending who knows you and your work well counts for a lot.
If you mean, "will an asst. chief of service's letter help me get a Derm spot w/ Step scores in the 70s?", it might if he's also the chair of the American Academy of Dermatology and he's your dad. I think that people focus far too much on who they get letters from and not nearly enough on the content of the letters. While it's true that getting 4 letters from your Path rotations won't help you much when applying for Plastic Surgery.
A bad (or even not great) letter won't help you, no matter who it's from.