Luis Valbuena came to the Cubs in 2012 off of the waiver wire, one of a dozen or so pickups the Cubs' had during the infancy of last season. Unlike most other wire pickups, though, Valbuena was a position player, and one that would see quite a bit of action in a Cubs' uniform in 2012.
Originally from the Mariners organization, Luis saw a cup of coffee with Seattle in 2008, until December 11, when he was part of a 3-team trade that involved an astonishing 13 players. Valbuena would spend a good amount of time in 2009 and 2010 with Cleveland, though he could never hit above .250/.298/.416 in either year. The Indians tired of his offensive deficiencies, and kept him in Columbus for the bulk of 2011. The Blue Jays purchased his contract in November of 2011, but they waived him and the Cubs picked him up.
In 2012, Luis spent the first third of the season in AAA, and he hit pretty well there. He earned an injury call-up after Stewart went down for the season, and stayed up the rest of the year, hitting .219/.310/.340, "good" for a .290 wOBA. He played above-average defense at the hot corner en route to contributing 0.9 WAR last year.
The Cubs did not upgrade at 3B this off-season, and Theo & Co. have decided to start Vitters in AAA next season. Valbuena and Stewart will be clashing for the starting spot; the loser will be the backup/utility infielder (if Stewart can handle 2B).
Valbuena is offensively-challenged in much the same way Darwin Barney is. Luis sports a career .283 wOBA and a .269 BABIP. These numbers suggest a lot of room for improvement, and luckily for Luis, there is some. A .269 BABIP is pretty low, and could easily climb to .290 or .300. Bill James projects a .295 BABIP for Valbuena next year, the primary driver for a wOBA of .320, which I think anyone would take from Valbuena (or 3B in general) at this point.
Valbuena doesn't have much power, especially from 3B, but he has slightly above-average plate discipline and he doesn't strike out more than average. He's not fast an a poor baserunner, but the Cubs' FO minimizes those parts of the game anyway, so it's not a complete worry.
Valbuena played pretty good defense at third last year, but before 2012 he had been known basically as defensively inept. He can play 2B/SS/3B/LF, all fairly badly, but passably enough. If the gains from 2012 are real (and defensive metrics take YEARS to have enough data to show trends), he could carve out a little career as a backup infielder that can draw a walk. If he can't, he's a AAAA guy.
Valbuena has three paths to sticking on a major league roster. Either he can realize his 2012 gains with the glove and stick around as a utility infielder, or he can bring up his offensive numbers to where you can live with his poor defense (still as a backup). Lastly, he can be on the Cubs, where we have no other real options. If Stewart never recovers from his wrist, the job is Valbuenas regardless of talent level. I'm not holding my breath that Valbuena will ever have a wOBA above .300, and I'm not holding my breath that he's suddenly become a defensive stalwart. Luis is organization filler; unfortunately for us, the hole he's filling in our organization right now might be "starting 3B."