Mike doesn't want a defensive assistant because he's totally happy with his "yes men" on the sidelines with him. MDA knows if he's forced to add a defensive coach he'll have to STFU 50% of the time and listen to reason.
Defensive coach: "ok guys bring it in... were going to..." (whatever the gameplan is)
MDA: "I don't understand"
D coach: "What is it?"
MDA: "Why isn't X player in your scheme right now?"
D coach: "Because Y player matches up better defensively"
MDA: "But X player can shoot really good"
D coach: "its ok Mike, just come up with a final play on offense"
MDA: "These guys already know to shoot it if they're open"
D coach: "oooo k" (inner confusion)
Hopefully at this point the D coach will be making trips to the front office to talk about the simple strategies on offense. (that's my pipe dream i guess)