For the Aegir project it would be usefull to create archives of just the site specifics.
Therefore --no-core was opted in
I've started some work on this in a sandbox, dump seems to be working.
I've added an extra testCase for this functionality,
The current method of restore is an issue though as drush_move_dir() is used which does not like an existing target directory. This use-case would need a 'copy' method.
The sandbox also contains a few commits with general cleanup.