While Bass' contract seems like a bad contract for the celtics, it's not for other teams. If Bass were on the open market, he would very likely get a bigger contract than 2 years $12 mil. So, if a team could trade away essentially nothing for him, that's a huge win for that team.
The problem with Atlanta, as stated, is this team is in much more need of a wing than a 4th big. Even if Brand is exclusively a center, when Millsap subs out, they can slide Horford to 4 and Brand at 5.
I think Bass should be considered a desirable player to have for any team in the East with plans on going to the playoffs for the way he can guard Lebron and Melo when either team tries to play small ball.
I think Atlanta should at least think about it, but they should really be going after a wing (esp if trading away Stevenson). Would they trade that same package for Courtney Lee?