Balance. Mills covered the opening swing bowler role in case of injury to Southee or Boult. McClenaghan covered the 3rd seamer short-ball specialist role in case of injury to Milne.From a distance, I would have thought there would be space for both of Boult and Henry in the squad. Not sure what the point of having a past his prime Kyle Mills in the squad was?
As mentioned above, Henry was on standby playing domestic cricket.