The Mariners' "defense" are currently on pace to finish just under 90 plays below league average. Based on Tom Tango's work, those ~90 plays results, on average, in 143 extra batters faced. At a league average 3.8 pitches per PA, you end up with 545 more pitches thrown by your staff as a result of below average defense. Given that not making a play inherently means there is now a runner on base, those are all higher-than-average leveraged pitches too. That means, extra stress. All in all, it's roughly an additional 5-8 entire games that the Mariner pitching staff has to throw because people like John McLaren are "happy" with his outfield defense.
NB: I actually wrote this before I knew Graham was writing his post. Yes, we care about defense.