Pearce has been quite a lot worse in the second halves in his career. Maybe that's partly due to all the injuries, maybe he just can't physically handle playing every day for whatever reason.
Regardless, this is an excellent signing at the price. Our bench right now has Pearce (1.5 WAR projection, could be higher), Upton (difficult to project, but likely 1 tp 1.5 WAR if used correctly and he hits more like he did in SD), and Barney (likely 1 to 1.5 WAR). Those are very valuable part time contributions for where we are on the curve.
Now if we could upgrade Upton to Dyson (we should have no problem trading Upton given he's basically free), we'd really be on to something with the bench.
Given the extra bat that kills lefties, I wonder if this is a prelude to possibly going after Moss? He's slightly above average defensively in the outfield, hits righties well, and probably comes at a reasonable price.