In the past, PB has not been able to allow the DH to move into the game into a defensive position to invalidate the DH and put the pitcher into the batting lineup. Alot of us players wish that was able to occur, but PB won't allow it (that is the game programming is not able to allow this type of switch).
I have used my backup catcher as the DH, but the risk is that I may need to put an outfielder in as catcher later in the game. So it is a finite risk...main trouble is you won't be able to PH for the catcher in a key time of the game if you are prone to.