I wanted to say something about this deleted scene from 15x19.
From an angel's perspective, calling another angel a good soldier was high praise and a compliment. Cas had previously said this about Balthazar, Ezekiel and Hannah.
Michael changed greatly under Adam's influence. I think he's begun to understand and appreciate people and their feelings. Just compare his two scenes at the bar, with Zachariah (when people died because of him), and with Lilith (when he only erased people's memories).
Honestly, I was moved when Michael said this phrase about Cas with such a sincerity and seriousness. I think he was simply expressing his condolences as best as he could. He understood Dean's grief because he had recently lost Adam himself. Besides, they truly lost a good soldier.
And then there was Dean's reaction, his visible heartbreak...
It's just my interpretation of this sad, emotional scene. I really wish this scene had stayed in the episode.