Before even looking at the trade, there is something you forget:
Al chose to come to Boston in free agency. I think he loves the organization and the city. You don't just throw him away 10 years after he came here.
I think this would be disrespectful to him after trying to make him come here. And I think this would be a bad signal to send to all potential free agents who might consider coming to Boston.
Keep Al. He's actually good.