I get why some folks don't like Anderson, but he does some things that are invaluable to the Spurs. He can guard all 5 positions. That's highly valuable on any basketball team. He's non-aggressive, except when he gets switched on a smaller player. So if a team is switching and he gets a mismatch, he can readily post up and score on smaller players. He is also a legit ball handler that can get the Spurs in their offense. When you have young guards and a shorty in Patty Mills, having that extra ball handler can be very valuable. He's also an unselfish passer (maybe too unselfish sometimes).
If he was a good three point shooter, matching him would be a no-brainer and really make the Kawhi mess go away. As much is Kawhi is better than Anderson, way better, if Anderson could shoot (especially from 3 point range), then he could be 65% of Kawhi, which is enough to trade him for assets and forget that he was ever a Spur. But he's more like 30% of Kawhi, which is probably not enough to pay the money they have to pay to keep him.