Yes, he is a good decoy, and yes, he makes big plays against mediocre teams. But elite teams are going to have an answer for him defensively. The game against the Jets is a perfect example-- Sure he caught a touchdown pass, but Tom threw two interceptions while trying to get to him, very likely because Moss demanded to be thrown to. When a player's ego becomes more important than the team, how on earth would you expect BB to keep the guy around? Being late for practice, complaining about his treatment in press conferences, confrontations in the locker room and on the plane ride... it was time.
It seems almost as if having Moss around has caused the defense to get lethargic because they feel like they don't need to play well, just good enough so the Patriot offense can finish it off. The offense is still going to be very very good, but now the defense is going to know that it actually needs to perform before this team will get anywhere.