Just to think outside the box here....
If Boston looks like they'll strike out this offseason with the restricted free agents like Harris/MIddleton and the unrestricted power players like Love, Monroe, Aldridge, etc...
There is SOME logic to trading for Joe Johnson.
Gerald Wallace (10.1 mil - 1 year) for Joe Johnson (24.8 mil - 1 year)
WHy for Brooklyn: Even after shedding 15 mil in salary, they are still over the cap next year (assuming they bring back Thad and Brook). But maybe saving 15 million dollars next year helps in their efforts to sell the team. Has that happened yet?
Why for Boston: I'll break it into 3 reasons.
#1 - We have something like 23 mil. This eats 15 mil of that. Again, if we fail at signing a top star, what difference does it make? We'd still have 8 mil in cap space to add someone like Asik or Biyombo. If the team dumps Bradley's salary (or a combo of players like Turner and Zeller), you might even have enough to still add a max contract deal. Trading Wallace for Johnson, perhaps, makes Boston more attractive to Kevin Love, right?
#2 - Theoretically, losing Joe Johnson hurts the Nets... which subsequently helps the Celtics since we own their draft pick next year. He's still one of the most talented players on that team and could definitely start for us at SF. ... If you ignore his contract and look at Joe Johnson the player, he's a got talent and can be a consistent scorer that we need. He made the all-star team in 2014 and can shoot. Wouldn't he fit well with Stevens pace-and-space system? Is there any chance that adding Joe Johnson to the Celtics hurts their team? Is there any chance that losing Joe Johnson helps the Nets next year? I'd guess no on both.
#3 - There should be no long-term negative impact towards it. He's a free agent at the end of the year. If you want to re-sign him you can do it for much less. The cap jumps 20 mil next offseason anyways.
The only reason not to do it would be if you are trying hard to free up cap space to sign two max contract players this summer... or Boston thinks adding Johnson stunts the development of young guys on the roster.