The most important thing that aran'gar/Halima did was free Moghedien from the a'dam, which meant that the two accepted, Elayne and Nynaeve could no longer pump her for information on how great works in using the power in age of legends were accomplished.  This was a huge problem for the forces of the dark one, because all of a sudden, long lost, war critical talents were being "Re-discovered" by the forces of the white tower. 

This was more important to the dark forces, than preserving the Enigma code machine secrets to the German U-boat fleet!