> I assume that if you integrate Babel and Webpack via webpack "loaders", some amount of work is saved (just guessing though)

The Webpack babel-loader just calls Babel with the source code, it doesn't reuse Webpack's AST: https://github.com/babel/babel-loader/blob/master/src/index....

It has to do it that way, since Babel could parse some syntax that Webpack doesn't recognise (like Flow or TypeScript annotations). Webpack loaders can even take non-JS input and produce JS output (eg. "svgr" takes SVG images and produces React components) so I don't think Webpack can even assume that the input is JS at that point.