They could use Kemba like they do Seth Curry when Doncic has the ball, and when Doncic is off the court or not handling they would have a much better option to give them some good offense. Instead they use JJ Barea or Tim Hardaway a LOT. Which is kinda scary to have in your playoff rotation. So much burden on Doncic....
The Mavericks already have an absolutely elite offense overall, so I don't think Kemba takes them to a title contender just because the flaws that hold them back aren't on that side of the ball. He would set them up to contend perhaps next season if they could shift their roster to shore up the D.
Of course Doncic is so young, no rush to contend this season.