It's a waste of a spot if the backup keeper isn't good enough to be the backup batsmen. No other side in the WC has taken a backup keeper, and the ones that have are essentially part time keepers like De Villers.
Now if only he could produce more of that!
No not really. He is their in the event Haddin has a serious injury & cant play along with above reason i gave before.
AUS unlike most teams IIRC since the 2003 WC have taken a back-up keeper to almost all major ODI tournaments (World cups 50/20 overs & champions trophy). But on tours a back-up keeper isn't picked. Since in world cup tournament if such a injury occurs to your main-keeper in crucial WC encounter i.e a QF of semi-final you dont want to have a part-timer as replacement, since flying out a back-up may not come in time.
AUS have been able to do that regularly since 2003, since you have always had alot of versatile AUS ODI batsmen & that trend continues.
Other teams haven't been as strong a ODI side over the last decade as AUS going into to WCs where they can genuinely pick a back-up keeper, given that they usually need that extra 15th spot to pick another bowler, batsman or all-rounder.
They basically pick 1 keeper & hope to god no serious injury happens them. Which historically hasn't happen in any WC tournament to any teams keeper.