I do not know why Nate is a bad defensive coach... - he can't go out there and actually play defense for these guys...
Let's face it - Sergio is a bad defensive player, Blake lost a step and is almost as bad, Travis is either on or off, Rudy is great in the passing lanes and he is better than Sergio - but he is not a good defensive player and for some reason (probably showcasing Sergio) - he does not play Bayless - who seems to be able to stay in front of his man. Batum is a very young rookie - he still makes some mistakes - but he is a very decent defensive player that will get better with experience.
I am willing to bet that once we start playing Bayless proper minutes instead of Sergio (or Blake), and once Batum gets some experience - our defense will look a lot better. Greg getting in shape and being able to rotate faster will also help - and when Webster comes back - it gives us another decent man to man defender (I suspect that Travis will be moved to backup PF where he is better as a man defender and we will see less of Frye - because Frye, let's face it - is just not a good defender).