I'm not much for identifying faulty swing mechanics, but with Arcia it was clear as day that he could not keep that back foot set for whatever reason and it was not surprising that he put up one of the worst full season performances in baseball at the plate this year. When a guy with defense that good still has a negative WAR, it's not good. He does seem to be putting together better ABs lately (he looked impressive in Game 163 and has put some good swings on pitches in the postseason so far, with great results to show for it). Sometimes he's up their hacking and his walk totals will likely always be low, but if he can get back to being the contact hitter he was and keep his mechanics in check, he should produce enough to stick at SS for the next 5 years and provide solid value as a 2-3 WAR guy in the 8-hole which is actually a very valuable player.