I would take him if you could get him and not affect the core of the team. This means you don't move: Bradley, Crowder, Brown, Smart, Brooklyn picks. And of course not Thomas or Horford.
if you're committing to IT - which you have to - you need the top of your rotation to be above average defenders. Bring in Carmelo and you weaken the D - more so if you give up Crowder, Smart, Brown (who's eventually going to be a very good defender) etc.
But...keep the core intact, add him, find a reasonably priced rebounder/defender to play the 4/5 against teams that currently cause us problems..why not? We'd be very, very hard to guard in the 4Q.