
It's a moot point: Step out of your door go _anywhere_ (except possibly
your mailbox). You will be "subject to some ad agency's 'poor taste'"
 

While I'm sure Sagan considers it sacrilegious, that wouldn't be
because of his doubtfull credibility as an astronomer. Modern, 
ground-based, visible light astronomy (what these proposed
orbiting billboards would upset) is already a dying field: The
opacity and distortions caused by the atmosphere itself have
driven most of the field to use radio, far infrared or space-based
telescopes. In any case, a bright point of light passing through
the field doesn't ruin observations. If that were the case, the
thousands of existing satellites would have already done so (satelliets
might not seem so bright to the eyes, but as far as astronomy is concerned,
they are extremely bright.)