This is actually a very interesting trade, depending what you think of Porter.
I think Green would actually be a great fit in Washington. They desperately need a better SF, and with Wall, Beal, Nene, and Gortat, Green wouldn't need to be the consistent scoring threat he has shown he struggles being. He can pick his spots, and they can ride him when he is hot. He also fits Washington's move towards higher character players, and has DC roots.
For Boston, they get to dump some salary, but they also get a young, talented player to replace Green in Porter. I think that is basically what you can hope for in a Green trade at this point.