Hidden value for MLB free agents

Adam Dunn and Carlos Pena are on opposite ends of the defensive spectrum. Getty Images, AP Photo

Carlos Pena and Adam Dunn don't seem that different. Both bat left-handed, play first base and filed for free agency. Both are what you might call "three true outcome" players: They strike out in bunches and have low batting averages, but they compensate with extreme patience and power.

Pena, 32, is coming off a poor offensive season, failing to hit above .200 (bringing his on-base percentage down to .325) and seeing his home run total drop from a league-leading 39 in 2009 to 28 in 2010. Meanwhile, Dunn -- who turned 31 this week -- has been a model of consistency, except for a disturbing 30 percent drop in walk rate this past season.