I read "Justin Langer may pick a fifth bowler" as "Justin Langer may try to trick New Zealand into picking a fifth bowler." Which in turn says to me that Australia is worried about struggling to bowl out the NZ batting if the MCG pitch is roadish, so one less batsman helps Australia roll NZ. Then again, the back-to-back Tests means Australia will have a weather eye on its Informed Player Management, and an extra bowler will ease the workload.