You have a very valid and true point. From a human perspective. Cas isn't human though. He's always been a foot soldier doing what the was told until he met Sam and Dean. I don't think strategizing with Dean crossed his mind because it seems, his intent was to protect Dean from it all.

Thinking about what you said, Dean would have been a good person for Cas to confide in and seek counsel from. I'm really kindof heartbroken for all of them right now.