Jeff Green's career hasn't turned out the way you would have projected early on, and more often than not he's resigned to being an idle perimeter threat on the Celtics. Having said that, he's better than Pietrus and Kleiza, and could add more consistent three-point shooting (like he did in year 2), and given how at this point trading Bargnani is addition by subtraction, this looks attractive.
From a salary POV, his contract is slightly below Bargnani per year but is one year longer, so this trade definitely has long-term impact which the Raptors have to seriously consider. The alternative is to trade Bargnani for a near-term expiring deal so you can truly shed yourself of him. In short, get Green if you see him as a nucleus to what you're doing in Toronto and not for the sake of getting rid of Bargnani.