What do you want?
I want to be able to write es6 modules enabled content scripts. To have the option to do something like
# in background page
chrome.tabs.executeScript(tabId, {file: "./path/module.js", module: true}, ...)
# or in manifest
{
...
  "content_scripts": [
    {
      "matches": "some-url",
      "modules: [ ... list of module files ... ]
    }
   ],
...
}

The scripts will be evaluated as module scripts and the resolution of modules will be in the context of the extension and not the injected page's context.

Why do you want it?
Because it will be awesome :-).
Since es6 modules landed and available for extensions background pages (at least very soon) having also support for using modules in content scripts will give all round nicer extension development experience and will make content scripts a first class citizens.