At the moment, js-builder package externalization works by externalizing one package per bundle. This works fine for most packages but it also needs to support bundling of multiple packages together so as to support package plugins e.g. moment supports plugins like moment-duration-format.
The effect of not doing this means that we can't externalize the likes of moment in the example scenario outlined above i.e. it needs to be bundled in all .js bundles (adding ~ 30 Kb gzipped). Not a huge deal atm because we don't have a large number of .js bundles.