It would be nice to have this conditional variable so that if you have created certain member groups (like a Site Editors group) who can see the site even when you’ve turned the system off, then you can insert a little notice informing them that the system is offline:
{if system_off}
<div class='warning'>Note to Site Editors/Admins: The site is currently not
visible to regular site members and guests, probably because the site
software is being upgraded. When the site is available to the public again,
this message will disappear.</div>
{/if}
This is more of a feature for a larger site that has a lot of editors, member support staff, etc., where one of the staff might not realize that the site is invisible to the public because s/he can see it.
It would be good to have a similar feature in the CP that would let you temporarily suspend all site modification privileges while the Super Admin is performing an upgrade, changing the database structure, etc.
One more related feature would be somehow preventing a search engine spider from de-indexing your site and/or indexing your “System Offline” page instead of your home page if the spider happens to visit during an offline period. I would think Google and other SEs might have some kind of meta tag you could put in the “System Offline” template that would essentially tell the spider, “We’re still here… just stepped out for a bit… please check back in a little while.”
