The premise behind this move is simple for the St. Louis Cardinals: limit innings for Dakota Hudson. By moving him to the bullpen instead of starting him, you can keep his arm warm and his routine in rhythm.
I like the move right now, and I don’t think it was prompted by his somewhat rough outing against the Brewers, but there are some issues with suddenly changing to a four man rotation. In particular, having a four man rotation changes the way the other four starters prepare for their appearances because of the change in rest days.