When Athanasiou played with competent players, he produced. When he was playing with defensive forwards, he didn't. Why play him on a defensive third line when he isn't a defensive player? If he can produce then why not put him in a position where we can maximize his production. Zetterberg will cover for him defensively, and will allow him to have actual possession time instead of being hemmed in the d-zone and then dumping the puck in or praying for a breakaway.
I know Athanasiou is a defensive liability, and he has some issues upstairs, but not every player is a great two-way guy and some zen samurai warrior that this org has a hard on for. We're in no position to be picky as to what type of players we want, if they're good and young, play them. If AA played on a better line, his scoring would more than make up for his defensive mistakes.