Why is Jason Castro always in the lineup?

After a rather quiet offseason, we learned that the lone big move the Twins made all winter was acquiring catcher Jason Castro. Renowned for his pitch-framing ability, his presence is supposed to aid the organization in improving a pitching staff that was consistently among the worst in the major leagues for the past half-decade.

Though its his defense that puts him in the lineup, the offensive side has been a bit more of a question mark for Castro. He does have the ability to draw a walk and he has rated as slightly above-average (career wRC+ of 109, so he’s been 9% above the league average) against righthanded pitchers, but he’s been atrocious against lefties (53 wRC+, so 47% below average). This has been mentioned plenty of times before.

And yet... there was Castro starting Opening Day against lefty Danny Duffy. And then he was in the lineup against lefty Jose Quintana on Sunday, and once more against lefty Matt Boyd on Tuesday. Oh, and the kicker is that ol’ 53 wRC+ against LHP Jason Castro was batting either 5th or 6th during all three of those games.

So... what gives? Especially baffling is that backup catcher Chris Gimenez has a 114 wRC+ against lefties, plus Gimenez himself has rated as an above-average pitch framer as well. Even if the argument is that Castro is the superior framer, he’s batted above the likes of Jorge Polanco (155 wRC+), Max Kepler (50 wRC+, though in just 142 PA), and Eddie Rosario (86 wRC+) in the lineup. Small sample sizes apply to Polanco as well, but my main argument is that Castro has proven himself inept against lefthanded pitchers while the other three mentioned at least have the potential to be better hitters than Castro.

This is a question where I don’t really know the answer. I suppose I’d have less of an issue with Castro starting against LHP if he wasn’t batting in the heart of the lineup. Chris Gimenez is as horrid against righties as Castro is with lefties so it seems natural that a platoon should be in order, but unfortunately the current roster construction makes that difficult when Joe Mauer should also be sitting against lefties. (It should be noted that Gimenez started at 1B for Mauer while Castro was catching on Tuesday against the lefty Boyd.) Hopefully with the pitching staff currently demonstrating that 13 pitchers hasn’t exactly been necessary, the Twins will be able to consider sending down a pitcher for a position player so the lineups can seem more natural depending on the hurler on the mound.