User blog comment:Zero62422002/Devil Fruit and Haki/@comment-68.43.155.67-20101011233400

Well, the Admirals are already strong as is. Besides, you have to learn haki through training, and I can tell you, Akainu is too bent on destroying pirates now, instead of training haki, and destroying them later. Besides, any haki that they would learn wouldn't be as strong as Luffy's 3 mastered hakis.