What it comes down to is motivation. There are two things that could motivate Wells to play well.
Wells has stopped speaking to the print media this year. An unusual move by such a prolific mouth, Wells was motivated through pride. He felt they weren't giving him enough credit and it's only gotten worse as the year went along (all those stints on the DL didn't improve the writers opinions for some reason).
Also, this is probably Wells last year pitching and he may be motivated to make a statement about his career as a whole by going out with a bang. Helping the Red Sox reach the promised land once again would certainly end his career on a high note.
Wells has a reputation for being a big game pitcher. While that 'big game' clutch performance has been missing recently, it's certainly a motivating factor in his makeup. Wells likes to compete, likes to go out there and put it on the line.
This is probably the biggest factor of all. His contract was written with all sorts of performance clauses, including a minimum number of games started. By getting him out there now, rather than a week later, David has a chance of hitting that number plus getting a couple of games at the $200K range.
BostonHerald.com - Boston Red Sox: All could be very Wells: Sox lefty nears return
Convinced that David Wells is ready to face major-league hitters, the Red Sox have all but settled on activating the veteran left-hander from the disabled list early next week rather than send him out on a minor-league rehabilitation assignment.