Because the pitcher is more important and if a guy, for whatever reason, performs better with a certain catcher, I don't break that up based on what hand the catcher hits with. Castillo was obviously way better last year with Casali. This year he needs to step forward and fill Bauer's shoes. I don't rock the boat there and let him do it. They may be able to rotate the others based on certain match-ups, but platoon advantage at the plate is way down the list of considerations.
I'm also happy to remove a lefty bat from the line-up. This shift killed the Reds. Winker, Moose and Votto are probably in there (though Votto sitting against lefties wouldn't be all bad), but Barnhart isn't a must have, and I think Casali is a better defender. Barnhart has a better arm, but that is very secondary when nit comes to Catcher defense. IMO.