Trades are best when they fill a void. This trade didn't fill a void.
Unless you consider CF or leadoff hitter a void, which some don't. I think Harper could have handled CF with occasional breaks and Werth could have been a serviceable leadoff hitter. But Rizzo wants Harper at a corner to take the strain of playing CF off of him and he believes (probably rightly so) that Werth belongs in the middle of the lineup, so he probably sees the trade filling two voids.