Is there a reason Yato couldn't summon Nora in the final battle of the first season of Noragami?
The whole cause of the battle was that Nora wanted Yato to use her again and be the god of calamity; he refused to go back to those times and that led to Nora kidnapping Hiyori. 

Even if it would disarm his opponent, it would make him a hypocrite because he wanted to prove that he was no longer a god of calamity.