Initially, the move appears to have been made with the intention of beefing up Boston's right-handed hitting, as Jackson comes with good career numbers against left-handed pitchers. Upon further inspection, however, the move becomes...puzzling. Jackson has terrible numbers the last couple of years, and they don't improve much when you only consider his at bats against lefties.
The idea seems to be to take advantage of Jackson as a utility fielder given his ability to play both first and left field, but it's hard to see how that's worth anything much. He's Darnell McDonald with a mediocre outfield glove who can do a passing job at first, though that will only come into play if the Sox lose a major player.
Jason Rice isn't exactly a big piece, to be sure, but the Sox have had the most success with their bullpen when they develop their relievers from within, and Rice had the potential to possibly be a decent sixth or seventh inning option down the line. This just seems like something for, well, nothing.