If Dwight opted in to the final year (so you would have 1.5 years), I would do that trade. Boston has enough 1st round picks and very limited roster space. I think Jones will be cheaper to re-sign than Sully and provides much of what Sully does. Howard is a clear upgrade down low and even in his reduced capacity would still be Boston's best player (and I'm not sure it is all that close). Acquiring Dwight is much more about the playoffs, than the regular season and he would be a huge help there (with all the days off).
So post trade Boston has a roster of
Starters
PG - Thomas
SG - Bradley
SF - Crowder
PF - Johnson
C - Howard
Bench
Smart, Jones, Olynyk, Jerekbo, Zeller, Rozier, Hunter, Mickey, Young with 1 open roster spot
Instantly the team is a lot better down low and defensively and despite Howard not being a great shooter, the trade would also improve the offense by now actually requiring someone to be guarded down low (Howard is still a guy you have to double team in the low block). Trade also opens up some more playing time for the younger wings/guards as Turner's minutes are gone. Next summer Boston still has the Brooklyn pick to add and still has some roster flexibility in Johnson, Jones, and Jerekbo (and Young may gone as well).
The reality is Boston doesn't need 3 more mid to late 1st round picks on the roster and any time you can add a clear upgrade at a clear position of need for picks you basically have to trade any way, you have to do it. Again though Dwight needs to opt in to his contract. I don't think you use that many assets on a guy that will leave in the summer, but 1.5 years I think is worth it.