Why has Conrad Smith been replaced at centre?, we were just starting to develop a good combination with Nonu!?

I feel real sorry for Smith. He's never played a bad test match and has been in great form lately but yet again has been left out of the starting 15. Although I believe Smith should be starting I can't understand why they have selected him as specialist cover for centre bench as there is plenty of other players in the squad who can cover centre such as muliani, tuitavake yet there is no cover for fullback - so there is no logical reason why we have a specialist center on the bench, maybe it was purely because they felt bad about the way they've treated smith!