There are definately pros and cons.I don't know about NZ but in Aus all the good phone deals include a locked phone which means that you can't use a sim card from another country while on holiday.The only allternatives are to pay to have your phone unlocked or use international roaming which can be very expensive.
Friend of mine wanted an unlocked phone purley to use on holidays so she purchased one from Cash Converters (2nd hand) and it has worked a treat both in Bali and Thailand
Cheers laurs