I don't see any particular reason not to add it, but there are some workarounds you can use in the meantime:

  - Build a cache of all site information with the site URL as the key, and then look up the `api_site_parameter` value (and associated information) that way.

or

  - Use the fact that the domain name can be passed as a valid value for the `site` parameter, in all API routes that require it. EG: [`/2.2/answers?site=unix.stackexchange.com`][1]
  
  Clean up the returned `site_url` to fit, i.e.:

        var api_site = network_user.site_url.substring(
            network_user.site_url.indexOf("//") + 2,
            network_user.site_url.length - 1
        );


  [1]: https://api.stackexchange.com/2.2/answers?site=unix.stackexchange.com