I'm gonna level with you -- while Garnett might make a good teammate and a great player, I don't think he is a good coach.
I watched clips of him "coaching" KAT, and was not impressed. KAT's high school coach taught him the things Garnett was trying to teach. It was obvious that KAT was listening because of Garnett's presence, but that doesn't make him a good coach.
Garnett's mind is so intense and disconnected that I think he would be one of the most frustrating coaches in basketball. He'd be talking super-intense to you, and because of his charisma you would want to learn and understand, but because his communication skills are lacking you wouldn't be able to. Then, when you do understand, you'd ultimately be disappointed because he was trying to coach you to do a drop step, which is something you learned in 7th grade.
I have no beef with Ainge not bring in Garnett. Yabusele, Ojeyle, and Theis should be learning from Al and Baynes.