If we're being honest with ourselves, how much chemistry did Billy Miller and Amelia have when his tenure was ending? Billy (the character) has been a total ass for the past few months, David is just continuing that. He's doing one better actually, he gives Billy an innocence and earnestness that Billy Miller never did. It's not David's fault that he doesn't have the history with working with Amelia that Billy Miller does.
This whole affair was written SOLELY as a contingency plan in case David and Amelia don't jive anyway, I just hope they don't break them up before we're allowed to see if they do. "Vibe" with Stitch and his throw-away character self is not worth sacrificing a Newman/Abbott relationship for.