It may be surprising to you, but home runs and batting average aren't the only things used to measure a player's value. Mauer posted a 7.9 WAR that year, the "aberration." He also had seasons of 6.4, 6.1, & 5.5 WAR. According to the Fangraphs explanation of WAR, anything above 6.0 is considered MVP level and anything from 5.0-6.0 is "superstar" level. The only times Mauer's WAR dipped below 5.5 was in his first full season (age 22) and the season where he played 109 games or less. The Twins didn't pay him for one season, they payed him because he's been the best catcher in baseball when he plays. If you want to argue against his health, go ahead, but don't pretend he's not one of the best catchers in baseball when healthy.