I will answer my own question from last night's game thread, when I wondered why Olivo was catching instead of Treanor.
"Olivo is playing great," Girardi said. "Not that Treanor isn't. I don't like pitchers to get too committed to one catcher in case a guy gets hurt."
Treanor is the better catcher. Olivo is the better hitter. It's strange how in spring training it was just the opposite.
I have no idea whether or not this is a semi-permanent arrangement or if it is a one time deal. I can guess this: if Olivo starts catching 4 starters his batting average will probably decline as the season wears on. For the most part, as catchers increase their time behind the plate their offense suffers as the season drags on, especially in the South Florida summer.
Olivo may be an exception. It will be interesting to watch, assuming the platoon is now 4 and 1, if it was just a one time shot - Treanor got to miss Oswalt and that is one lucky break for him.