Gearin is a ROOGY at this point, he's had extreme splits in the majors. Granted, that's SSS. But there's a reason behind it to back it up: he doesn't have a pitch to use against lefties.
He would be a great complement to Sherril, but Fredi thinks of all relievers as entire inning relievers. This is probably Wren realizing Fredi would misuse Gearrin like he has misused Sherril. Plus, bringing up another one-batter reliever wouldn't help lessen the load on o'ventbril as much.