Nothing.

The posts here are attempting to answer a question less simple and further removed from one regarding inherent danger, and that is: "Why should I be required to procure an ID to fly?" IOW, what purpose does showing ID serve? (And to whom?)

The overwhelming consensus seems to be that it serves a purpose for the airlines' bottom line to some degree or other, but it serves nil-to-none in terms of airline security. If someone can prove that it does, we're all about enlightenment here.

So far, no such argument has made any sense. The mere acting of flashing a piece of picture ID that matches you in person doesn't do anything in and of itself to prevent bad things from happening to a commerical airliner.