Picking up the one-year option, although at a higher price per year than a 2-year deal, would leave the Sox with the needed flexibility in the future.
For example, the Sox could sign Dunn long-term, and move him to DH in a year if they acquire Gonzalez/Pujols (and probably keep him at first if they acquire Fielder - but I don't know how willing he is to accept a future like that). Or, they could sign Werth and a stop-gap at 1B/3B, and then move Werth to DH, Youk to third, fill in right with one of our emerging prospects, and pick up a 1B. Or slot an outfielder at DH and keep Werth in right, idk. Or, rotate V-Mart a substantial amount between C/DH in two years. Or trade for HanRam and put his glove at DH.