Ben Oglivie, Duane Espy, Dave Magadan, Merv Rettenmund, Wally Joyner, Jim Levebvre, Randy Ready and now Phil Plantier. These are all the hitting coaches that have come and gone since Rettenmund initially left the Padres after the 1999 season. Some didn't even make it to the offseason before moving on, but Plantier rode out this rough season before getting removed from his position. This has been a standard play in the Padres playbook with each result almost seeming worse that the last (if you just go by the hitters' production).
There is no reason to think that this will make the team better. Alternatively, it probably won't make them any worse. What strikes me as weird is that this is the only move on the big league coaching staff. Without some sort of explanation it seems like a scapegoat scenario. The team actually had two hitting coaches, the second being Alonzo Powell, but only fired one of them. One would assume that if coaching were to blame then both should have a share, but that is not the message being sent here.
Perhaps you can tie this in with the other organizational changes being made. Parts of the player development staff were let go, but that seems a bit separate from moves made to the major league staff. Perhaps there is a whole philosophy change at work here. But without any word of that going on, the conclusion most will draw is that the hitting was bad so the hitting coach got fired. Nevermind that most experts would tell you that the hitting coach can only do so much. This isn't football where the offensive coordinator calls plays, puts guys into the right position and makes sure everyone knows his system. Hitters have 99% (or some high percentage) of what they need to succeed already in place and the hitting coach is more of a mentor than a difference maker. Changing that role doesn't change the team's offense, so why make a change?
On top of all of that is that Plantier came to the job with a reputation in the organization for helping to develop its young hitters. He was a Jed Hoyer hire. A GM with a reputation for player development, just like Preller does. If one were to give Preller the benefit of the doubt and think that he is making changes that help player develop, then it still leaves you scratching your head a little on this one. In the end though, there's not much additional info to go on here. But on the surface it seems like the same tired move of rearranging deck chairs on the Titanic.