yeah, I know he already used it before Higueras but I am pretty sure he's been using it a lot more since 2008, especially on clay. I wonder if this might have been encouraged by Higueras (someone Roger hired to help him on the clay).
Roger still doesn't use the drop shot much on hard courts. After winning Cincy this year, he mentioned that he specifically used the drop shot more on clay because it was harder for his opponents to get to it on that surface.