We had a discussion on IRC #ubuntu-x right now, and the consensus was that by default, g-s-d should not turn external screens on or off, but leave it the way the X server has set it up. If the user later configures things on/off these settings will be applied next time.
The issues in this bug reports reveal some bugs in g-s-d and xorg/drm, but they should be fixed independently of this turn_on_external_monitors_at_startup question.
We had a discussion on IRC #ubuntu-x right now, and the consensus was that by default, g-s-d should not turn external screens on or off, but leave it the way the X server has set it up. If the user later configures things on/off these settings will be applied next time.
The issues in this bug reports reveal some bugs in g-s-d and xorg/drm, but they should be fixed independently of this turn_on_ external_ monitors_ at_startup question.