He's just too exploitable on D.
So far, the Al Horford + Aron Baynes front court has given up a about ~90 points per 100 (depending on which site you check, the number ranges from ~89 to 93) and all of our best defensive lineups overwhelmingly include Baynes on them. Per nbawowy.com, with Baynes 'ON' the court, we've given up just 97.6 points per 100 (393 minute sample). With Baynes OFF the court, we've given up 106.3.
There is no getting around the fact that Baynes' minutes on the floor have been a huge part of why we have the NBA's best overall defense.
He's excellent when there's a plodding big man to cover and he protects the rim very well for someone who isn't a shot blocker.
But there's a reason he played all of about 5 minutes vs GSW, they just destroy him in switches and backdoor cuts.
Having Morris out there
Having Morris out there ... with the starters at the start of the game was when GSW jumped out to a big lead. During that time, Zaza (a big plodding 7 footer) was on the floor. We clearly struggled the most in the entire game during that stretch going -4 as a team while Morris was on the floor here against Zaza.
For some bizarre reason, Brad brought Baynes in at the end of the 1st period, after Zaza sat down and GSW went small with Green & Iggy as the front court. So yup, Baynes suddenly looked slow. The C's were another -4 during that short stint with Baynes against those two quick guys.
So that game was really poor big man usage by Brad, imho, to start the game.
And again, to start the 3rd period, Zaza was on the floor and so was Marcus Morris and we again sucked wind, going -10 during that stretch with both Morris and Zaza on the floor.
Morris sat down and Baynes came in to cover the last bit of Zaza's run and this time Kerr put McGee in. So during Baynes' second half run, he actually mainly faced Zaza or McGee, two 'plodding big men' and wuddayaknow? The C's went on a +15 run during that short stretch.
Golden State overall posted a +16 against us while Zaza was in the game. He was only negative for a very short segment overlapping Baynes.
Now, there were lots of other factors that affected the plus-minus flows in this game. Curry shot poorly. Jaylen put on his MVP cape. But the above are how things went in regards to the Morris and Baynes substitution schedules.
My point is never that Baynes should be out there on the floor all the time, especially against really quick front courts like, say, Green & Iggy. But the fact is, he's proven _very_ effective against the very large chunk of minutes that other teams put their own big plodding guys on the floor. And most teams -- including GSW -- still use their big plodding guys a ton. And usually (especially) at the start of games. We need to do so as well.
And I think a big additional factor on why starting Baynes has been so effective is that it puts Horford against smaller 4s. Horford has excelled this year when playing the 4. It allows him to roam more on defense knowing we have a big guy defending down low behind him. And on offense, he's been able to really take advantage of size mismatches this year. His TS% when on the floor with Baynes this year is 647%. When on the floor with Theis (smaller sample) it has been 68.3%. When Al is at the 4 and popping out to the perimeter for high screens and 3PT shots, we still have a 'true big' down low to block out & contest for rebounds.