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(1) Ackley has spent his minor league seasons learning second.
(2) Ackley's value would be diminished at a bat-first position like 3B. His BAT wouldn't be diminished, just his value.
(3) Kennedy, while mostly used as a MIF, has experience at 3B. So does Rodriguez. Being temporary solutions, because of the bat-first nature of the position, it makes more sense to employ THEM there than Ackley.