It seems that Miller's problems at the plate can be traced to issues with his swing, which can oftentimes appear stiff and mechanical. He inside-outs everything, leading to a lot of weak contact to the opposite field, and rarely squares the ball up to make solid contact. While he's clearly strong enough to hit for at least some power, when he gets fastballs right in his wheelhouse that he could be driving out of the park in right, he often ends up hitting a dribbler to the shortstop. I can't recall the last time I saw him pull the ball with any authority. Whether it's a matter of timing, or the positioning of his hands, or his stance, his swing has basically remained the same, with no obvious adjustments, and the numbers have continued to decline. A competent hitting coach would be able to diagnose these issues and help Miller make the necessary corrections in order to get the most out of his ability. Unfortunately, that's a luxury we don't have.
If Miller is in the lineup, however, much of what he brings to the table is wasted if he's being used as the DH. He's a very good defensive outfielder at either corner, and better in the field than the two starters.