Chris Stewart has a good defensive reputation, and was one of the top defensive backstops in the minors last year, throwing out 52% of basestealers, but the acquisition of Toby Hall to back up Pierzynski (both signed through '08) meant that Chris Stewart was going to run out of options before he permanently found his way onto the White Sox active roster. Kenny Williams flipped the light-hitting defensive specialist for volatile yet projectable reliever coming off a season with a poor ERA and walk rate. Perhaps most importantly, he freed up a spot on the 40-man roster in the process.
I'm probably reading too much into the open roster spot -- I've advised some of you against doing so in he past -- but as the Sox current roster stands, they still have only 1.5 outfielders, 1.75 tops.
More than likely, this has nothing to do with our current outfield situation, and more to do with Williams and his scouts liking what they've seen from John Lujan. He was probably one of the arms they were looking at in advance of the Brandon McCarthy trade, but the Rangers were reluctant to part with him because of his reported 97 MPH fastball. Kenny offered the Rangers a player with more value to them than to the Sox... and a deal was struck.