For me it is the single biggest feature addition. Just changing one file would require rebuilding the entire site in Jekyll. Something similar to a Makefile should be in place, which can track dependencies and rebuild only the affected files.
It's static only relative to the output. You can readily write dependencies between your source (mdown, partials, whatever) files, just like your binaries/libraries depend on .o files, which themselves depend on .c and .h.
I fail to see how changing a typo in an article's content would require the whole thing to be rebuilt. Even adding a page would not require rebuilding every single page unless the new page appeared in a global navigation. Building the dependency graph automatically could even turn out to be not that hard.
