I'm not saying thats the only reason. However, I can't see much more of a reason to bring back a disgraced mediocre-at-best hitter (Yes, I realize he had 550+ HR's, but would you hire Adam Dunn to be a hitting coach?). Also, I think it was another move to keep La Russa on board. I'm willing to bet Big Mac does not see more than 5 years in this position.
Not to mention, I hate to think that the guy who "broke" Holliday's form was McGwire. McGwire was the one who tried to teach him to go without his high step in his swing, and Holliday's first half was abysmal. Once Holliday went back to his old style, with the high leg kick, he was back to normal.
So far I'm very skeptical, but I have already been proven wrong once this year. See: Josh McDaniels