A differentiation needs to be made between moving someone into an unfamiliar position during a game, when the coach is restricted by only having 23 players, compared to a coach knowing on a Wednesday that he's short in a position, 15, and has the choice of putting Bertie in to start at 15 or James, I'd go with James.
If during a game Dai has no other option due to a catalogue of mid game injuries, then fine, move Bertie to 15.
I can sort of understand why Dai started with Miller at 10 when Sopps, Searle, Gopps were injured, as Miller had played there before and did a good job. Its just a shame Umaga didn't even get a few mins on the pitch in the 3 or 4 games he kept the bench warm.
I wonder if that decision by Dai has come back to bite him with Umaga's future at Wasps still a mystery.