Although in world war z (book not movie) Max Brooks had some wonderful reasoning for the slow zombie. I think WA fast and smart zombies, are better for an action audio drama. Brooks' approach also didn't require much in regards to action sequences.

The other thing I think is important to remember is Datu's comments if taking a back seat to his own body. I've always thought that the zombies themselves are operating on a more primal instinct than 'brains!' kind of thing. I think most of the regular zombies are like dogs, they need direction from the smart ones, and the smartest one we know of is Ink so he has become King over the other Smarts.