Any dignified and professional AG would be interested in such information. To declare flat out that "there is nothing to investigate", or however Holder phrased it, amounts to gross misfeasance or malfeasance in office.
Taguba apparently has a well-developed conscience. He is able to tell the difference between right and wrong. He knows that torture is an international war crime, and he knows that his government once upon a time prosecuted torture.
Some humans do not have a functioning moral compass, and Holder is one of them.