Birthers provide excellent cover for the White House:
Whenever there is too much heat on the WH, they stir up a few more birther stories. This refocuses the media and electorate on how batsh!t crazy these people and the Tea Party are, and divides The Right/Republicans. Its like a gift from heaven for Axelrod. They do not release more documents because they do not want to eliminate all doubt - small glimmers of doubt on this issue keep it alive, and make it a very useful tool for BHO's administration.
I expect them to release his birth cert 10/'12, but only after a crescendo of doubt and conspiracies are created (by the White House in part).