My thinking on that

We don't know if David Ortiz will be back in 2008 or not. Assuming he's done for the season, Boston needs another bat to protect Manny. Jeff Bailey won't cut it. 
 
Look at it this way, as a hitter, Barry Bonds is a perfect fit. He'd replace the bat and presence of Ortiz like no other hitter. He'd also come cheap as nobody wants him. If the Sox traded for another bat (Griffey?) then they'd lose a top prospect because everyone knows they need to do something.
 
He'd fit in, too. Think of Randy Moss and the Patriots. They lost, I know, but the potential off-field issues never happened.
 
Bonds would leap at good PR from a possible World Series run.