It's actually not common sense at all. It's an assumption made with nothing to support it. It doesnt' make any sense at all to assume one player will play worse, and have worse stats, because Milton Bradley is on his team. It's a really stupid idea actually, with no support. just because it's hard to quantify, doesn't mean there is no support. in THT Live one of the guys estimated that mlb team values chemistry at about 2 wins, IIRC. He based this on a very small sample of players that get waived or go unsigned are usually at 2 WAR. That's just what I remember from an article written in August or September.