Green is valuable. To think he's not is silly.
Is Green really all that valuable? He's fairly productive as a wing scorer -- most wings who score at a higher rate cost more, are on rookie deals, or else are pretty poor defenders.
However, he doesn't do much else well. He doesn't rebound much, he's not a playmaker, he doesn't force turnovers very often, he doesn't protect the rim. He's also not even a particularly efficient scorer.
If you want a wing who can give you 15-16 points a game without dominating the ball and also play adequate to pretty decent perimeter defense, Green is a decent deal at $9 million a year.
The thing is, your $9 million would probably net you a more valuable player if you spent it elsewhere.
DeMarre Carroll, who is being laughed at on this forum as the prospective replacement for Green in a rumored trade with Atlanta, is far more of a bargain than Green at $2.5 million a year -- and he has a substantially higher career WS/48.
Honestly, that's why Hawks fans would be right to feel that the rumored trade is a bad one for their team, especially if the Celtics asked for a pick in addition to the package of Carroll and Brand.