Jones was once a highly-regarded prospect, but he recently turned 26 and hasn't done much at AAA. Projection systems still tend to like him, probably because he doesn't have huge holes in his game (he can hit for a bit of average, has some power and controls the strike zone decently), but the same has been true for Brandon Moss. Like Moss, Jones hits lefty and doesn't really have a standout skill at the plate. Also like Moss, Jones is supposed to be strong defensively in a corner. Jones has more experience than Moss in center, but the results haven't been particularly good.
Moss' fate already seemed to be sealed by the Ryan Church signing, but Jones' addition would seem to make it even less likely that Moss will be around much longer. It's a bit strange that the Pirates are adding a player whose skillset is so similar to Moss', but Jones was more highly regarded as a prospect than Moss was, and Moss had his chance to prove he wasn't good enough. Jones hasn't, having only received 148 at bats in the bigs spread out over the past three seasons (and actually playing reasonably well in them).
Of course, it's possible that Jones will have no impact on the big-league bench, at least not right away. The Pirates could just send Jones to AAA and hope he catches fire there. They don't have the option of doing that with Moss, who is out of options, or with Rule 5 pick John Raynor. But Jones' skillset conspicuously resembles Moss', and if I'm Moss, I'm probably concerned that that means something, whether that's being cut or traded.
Jones' acquisition fills the 40-man roster, but the Pirates are apparently waiting for confirmation from MLB before officially adding one of their more recent free agent additions, so they'll have to remove someone from the roster when that comes through. You can speculate about who that might be. Someone like reliever Anthony Claggett might be the best guess, but if I were Moss, I might be worried.
UPDATE: The Post-Gazette, linked above, now reports that Jones may not be a Pirate for long--since the Pirates were second in the waiver queue, they might have claimed him for another team who wanted him, and may now be trying to work out a deal where they send Jones and another player to that team for one guy they like better. Stay tuned.
UPDATE II: The Pirates have cleared a roster spot for Ryan Church by designating Steven Jackson for assignment. Which is a little strange, because I thought Jackson would get a shot at a bullpen job this spring, even with all the remodeling over the past week or so. Jackson isn't nearly as good as his ERA with the Pirates suggested--his K:BB ratio with the Bucs was awful, and for all the talk about him racking up groundballs, it's worth pointing out that he didn't, you know, actually really do that. But he did keep his head above water, and he strikes me as pretty clearly a better option than Claggett, who's still on the roster for some reason. Most of the other pitchers on the roster do have some reason for being there, so one wonders what the Pirates see in Claggett. The Pirates also still have Argenis Diaz and about eight trillion outfielders on the 40-man.