As noted by BadAndy in the FanShots, the ex-Nat will compete for a job as a lefty specialist in the bullpen this spring.
Slaten throws a 90-ish fastball, a slider, and a changeup. He's mostly been used in a situational role during his ML career, with good reason: Over his career, he's held lefties to a solid .241/.304/.361 batting line, but righties have smacked him around at a .297/.391/.473 clip, with more BB than K. If Slaten does end up making the 25-man roster (and Daniel Moskos isn't exactly an insurmountable obstacle in his path), Clint Hurdle will need to handle him carefully in order to maximize his value to the club.
It's also worth noting that Slaten missed a significant amount of 2011 with ulnar neuritis in the elbow of his throwing arm, and that after returning in September he put up an 11.25 ERA in eight games. Did his injury play a role in his late-season struggles, and if so will he be back at full strength this spring?