First, the glove: Chris Carter is bad defensively. Chris Carter is considered by PECOTA to be the worst first baseman in MLB (by projected FRAA). Chris Carter is considered by scouts watching his defense to be an abomination, possibly of a magnitude portending the apocalypse, with mankind's only hope of salvation the decision that he can no longer play left field.
But two points:
1) Over a whole season, he'd probably only cost his team around nine runs. Defense at first base isn't as critical; and
2) Carter is better than Casey in projections even after that defense is considered.
***
Why would the Diamondbacks dump Chris Carter? They didn't "dump him," they acquired a good relief pitcher for him. Furthermore, they've got Conor Jackson at first base--Jackson had the scouts drooling a few years back, and the Diamondbacks organization is committed to giving him a longer chance. Frankly, Jackson could still easily develop as another Cecil Cooper...I see first base in Arizona as blocked.
Yup. I might be.
Carter would've been the backup for, essentially, both third base and first base. Given that Youkilis struggles with more playing time and that Lowell is aging, 200-300 PA would've been likely, IMO. We'll see how many PA Casey gets.