Howard has become a lot more stiff the last two years. Up and down, he is sturdy, but side to side, he is not.
He ranks in the 10th percentile guarding big men on the pick-and-roll. That's 10th.
He is one of the best post defenders (97th percentile) in the league. He is also a good ISO defender (60th percentile). His rebounding rate is near tops in the NBA (75% rebounds per chance adjusted). He is also defending the rim pretty well allowing 45% shooting at the rim. And overall, players shoot 4% worse against him.
There is no doubt that he would help our weaknesses. However, so far, as Ainge builds, he has built a team that fits CBS system. Howard does not fit that system. Unless CBS is committed to building around Howard's weaknesses, that move might frustrate CBS.
If CBS was completely on board and the price was a Johnson, Zeller, and one (or two) future non-Brooklyn first, I'd be all-in.