A flag to disable copying static files may be useful. Enabled by default so no ones systems break.
For local development it would be nice to disable and only have hugo build when content or html change. Well any file for that matter except for static. Or maybe a flag which accepted a glob of files / paths would be useful to prevent hugo from rebuilding when modified.
When i’m working locally i’d like to exclude hugo from copying static files to speed up the build. Our build system copies individually changed static files to our public folder and refreshes the browser which makes for a little faster development.