Batting numbers are easy to see and quantify. You can quantify defense to some extent, but there's no single stat that works. Also, defense is often more team than individual. So if you put a really good defense behind a Priester or Quintana, who get lots of ground balls, your winning percentages over the course of games will be pretty good.
Ortiz has been a key to our success this season. But I hope that in playoff ball, when each game is so important, Murph will sub out Ortiz for Mona in critical hitting situations. Mona is hitting .288 right now, and he's proved he's really good in clutch situations.