I suspect the reason for the the expired rule is due to the expectation that ID security features will continue to improve and by requiring a current ID, the presumably harder to duplicate features will be needed for a passable forgery.

However I personally think it is all irrelevant and only serves a purpose for revenue protection for the airlines (it make it harder to sell an airline ticket that you wont be using).

To demonstrate why it really serves no protection against a real terrorist (who is intelligent, well-informed and motivated), here is how the terrorist (even if he is on the no-fly list) can get onto practically any domestic flight:

Step one: He buys a ticket in someone else's name for whatever flight he is interested in (let's say SFO-JFK if he wants to make sure there is a lot of fuel on board). Ideally the name will correlate well to the terrorist's apparent ethnic background.

Step two: To really cover his tracks, He buys a second ticket in yet another name (that similarly matches his appearance) for a commuter flight operating out of the same terminal that departs at a similar time (some people suspect that small commuter flights get less scrutiny than transcons).

Step three: He prints boarding passes either at an internet cafe or at the kiosks at the airport.

Step four: He presents himself at security with the second ticket and no ID. He undergoes secondary but is admitted to the sterile area.

Step five: He tosses the ticket that he used to get through security, and uses the first one to get on the plane he wanted to get on.

He is on the plane he wants to be on, and its going to be decidedly hard to figure out who he is after the fact. He can also further obfuscate his trail by flying around on several tickets (each in a new name) before getting to his target flight. As long as he stays airside, switching identities is as simple as pulling a new boarding pass out of his pocket.