Internet connectivity is almost a given these days and some day it may even be ubiquitous. But that day is not today, so, when you've switched a service like Steam to work offline in anticipation of downtime, only to have it ask you to reconnect at an inopportune time... well, it shouldn't be doing that, actually.
According to Valve representative "henryg", Steam's offline mode was not designed to require one to reconnect periodically and was always intended to be left permanently on if the user wants. Not that it works like that, but apparently it's something Valve is working on.
Here's the full post, which was in response to a user who stated (incorrectly) that Steam was "designed" so that offline users have to signal the digital mothership every two weeks:
This is not actually true -- Offline Mode is designed to be indefinite. You can't access any of Steam's online features such as friends lists or saved game synchronization, of course, but the client should allow you to run in Offline Mode for as long as you like.
That said, there are many components involved in Offline Mode, and some of them have known issues and bugs which we are continually working to improve. We're aware that it doesn't always work as flawlessly as we want it to, but please keep reporting bugs with Offline Mode. It is not broken 'by design'.
Hopefully we can look forward to a permanent offline mode for Steam... probably around the same time as Half-Life 3.
How to have reliable offline mode on Steam [Steam Community]