Both Loperfido and Barger are going to suck against Rodon's slider so they must feel that using Barger as a pinch hitter later on is a better use of their talents. Plus, Loperfido is better defensively.
Ideally, they should just bat Barger 3rd in the lineup every day. It would break up the righty mainstays (Springer, Vlad, Bo, Kirk) while making it tough to pick on Barger with lefty relievers late in the game. The Book says it's not as important of a lineup spot as it would seem anyway so it probably doesn't kill us in games like today with lefty starters on the mound.