But it's not only about maximizing Mauer's plate appearances, it's about maximizing the potential for whatever his replacement does. Example 1: Mauer plays catcher and Player A plays first base. Mauer = .320, .400, .450 Replacement Level Player A = .290, .330, .440 Example 2: Mauer plays first base and Player A catches. Mauer = .340, .420, .470 Replacement Level Player A = .240, .290, .410 Clearly example 1, where Mauer catches and we insert a league average 1B into the lineup gives us more production out of the two positions. In example 2, even though Mauer's individual numbers go up, the total production out of the same two positions goes way down. This is why Mauer has so much value as a catcher.