The backup catcher needs to be on-call for far more than 16-20 starts a season, even with an All Star as the starter, which we don't have. Coincidentally, we have such a comparison to look at - 2008 Joe Mauer was rested for 28 starts, and as a 25-year old that was his high water mark as a starter. Appearances by a third catcher, if that's what you really have in mind, in fewer than two dozen games doesn't move the needle on winning lots more games, and amounts to more of a publicity stunt if done using Mauer. Which is all aside of the statements from the team that Mauer's health isn't going to be risked by playing behind the plate.