Umina, the logic for this is very simple, it is not in the least bit ridiculous, more a Hobson's choice.
Is DWZ a better fullback than Peachey? The answer is yes.
Are the Panthers a better team with DWZ at fullback and Crichton or Phillips on the wing vs Peachey at fullback and DWZ on the wing? The answer is no.
The Panthers are a better team with DWZ on the wing and Peachey at fullback, much as I would love to have him there, our back up wingers are just not up to it. And forget the idea of moving Whare to the wing and Peachey in the centres, that changes 3 positions. Peachey to fullback, changes one position. Simples.