I've been saying this all along but the Time article refrenced below lays it all out nicely. If they can get rid of Gonzales maybe the flood gates will open and the dominos will begin to fall.

JSP
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If cabinet members were perishable goods, Alberto Gonzales would have passed his "sell by" date sometime last spring. Since January, when he first faced sharp questioning over the firing of U.S. Attorneys, the Attorney General has earned disastrous reviews for his inconsistent testimony, poor judgment and for appearing to place loyalty to the White House above service to the public. By June it was hard to find a Republican willing to defend him.


source

Gonzo has lost all his credibility, so keeping him on serves no purpose other than to keep him quiet by protecting him under executive privilege. He must have some pretty damning info on Bush if he is going to this extent to shield him.

Even Gonzo's investigations from this point on will be suspect. His office has lost it's credibility. He's a liability to the Justice Department.

He also is used as a focus object, as long he is still there the reps. will concentrate on him and other stuff will go virtually unnotoived.