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They want Ackley to get his "reps" so that he doesn't make a few bonehead plays in the spotlight and damage his progress defensively (and offensively) by starting to "play scared" with the glove.
Wanting to get him "real-time" "game-speed" decision-making skills in a variety of permutations (that you can't re-create in drills) is not the same as saying "we still need to figure out if he can play second in the majors."
Plausible, anyway. But still probably only part of the "cover story." :)