I understand the frustration with Chone Figgins and the infatuation with Adam Kennedy. There's over 200 OPS points of difference between the two currently and the team typically looks anemic at the plate. Nobody is satisfied with Figgins' performance, almost certainly not Figgins himself, but replacing him with Adam Kennedy is a bad idea. It's a knee jerk reaction that has no way of standing up to even the barest rational investigations.
By my own park-adjusted wOBA (which includes reaching base on error), Chone Figgins has a wOBA 59 points worse than Adam Kennedy's right now. Assuming that stayed constant for the remainder of the season, and that there is another 450 plate appearances to be had by the everyday third basemen, that difference in batting lines would amount to 23 runs of difference.
I hope that by now I do not have to go over the litany of reasons to expect Adam Kennedy to hit worse going forward than he has so far and for Figgins to do the opposite. Kennedy's current rate stats are among the best of his career and that seems unlikely to hold up over another 450 plate appearances. For one, if he assumed the starting mantle he would have to face all pitchers. As it stands, Kennedy's batting line has been achieved through a rather fair amount of platooning. Kennedy has faced a right-handed pitcher in 79% of his trips to the plate while Figgins has faced one in only 67% of his. There is almost no way to say that Adam Kennedy would continue hitting this well as a full time player.
Of course, hitting is only part of the battle. Fielding is another one and there Figgins has a distinct advantage. All the fielding metrics agree that in his halfish season at third base, Adam Kennedy was worth about five runs below average with his glove. Over the remaining amount of games this season, he'd project to be -10. On the other hand, Figgins at his current defensive rate would be +3 runs to average for a total spread of 13 runs between the two.
There's some other miscellany (baserunning) to consider, but nothing major that would dramatically shift the paradigm. What you get is that if you assume no regression for current performance --and again, that's preposterous to assume and regression favors Figgins-- Adam Kennedy would be worth about one single win more than Chone Figgins if he started in place of Figgins at third base for the rest of the season. That is really the best case scenario for Kennedy supporters and no sound thinking person should believe in that best case.
And even then that's a bad call. Adam Kennedy is 35 and not signed with the Mariners past this season. He's a role player who will play out his career getting gigs like the one he's currently held. Chone Figgins is 33, but more importantly has years left on his contract with the Mariners. Despite what some people will claim, Chone Figgins is not a sunk cost. To say so is fundamentally misunderstanding what the actual definition of a sunk cost is.
Chone Figgins' cost to the Mariners in 2012, 2013 or 2014 could still be avoided. It's not a likely chance, but neither is it likely that Chone Figgins' BABIP remains at .230 or that Adam Kennedy continues to hit for the highest isolated power in his career. Playing Chone Figgins is the Mariners best chance to extract value from him and if they want to win in the future, then getting some value from Figgins either with the team or in shedding some of their contractual commitment is the best bet. Playing Adam Kennedy over Chone Figgins is a win-now move that even under the more favorable of assumptions to Kennedy, does almost nothing to help the team win now while hindering their ability to win in the future. It's the exact opposite of what this team should do and the opposite of what fans should be clamoring for.