Typically I would think such functionality doesn't belong in Babel, and should be delegated to build tools like make, Gulp, etc. However Babel does include minification, for example, so it looks like project philosophy isn't that minimalistic.
Skipping compilation of source files that are older than their output counterparts sounds like a simple thing to implement and would make my development process much better.
I'm aware of watch, but I'd prefer the above solution as it better suits my workflow.
Any chance this could be implemented or included if I send a PR?