The fact that Steam requires you to pop online once every few weeks is a bug rather than intentional design according to an
official forum response
. "There are many components involved in Offline Mode," writes Valve poster Henryg, "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'."
He also mentions that Valve are in the process of converting every game on Steam to a new authentication procedure. "Some day soon, once this work is completed, we will eliminate the old authentication system (represented by the ClientRegistry.blob file) and Offline Mode should immediately become much more robust."
Steam's offline mode has never worked quite correctly, to the irritation of many, including the OP on this forum thread. The poster works five week shifts offshore, but can't access Steam games for more than a couple of weeks because of the impromptu online authorisation issue. Henryg suggests that "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."
A fix is on the way then, but why have Valve kept it so quiet? Henryg says "We have no community managers or PR people, so all the time that we spend on forums is taking away from development time." Ah, I see. well in that case GET BACK TO WORK HENRYG.