I honestly can't seem them writing out Cas. He's just far too popular. And from what I'm reading on twitter (and yeah, I only read a very select group of people) Mark P isn't particularly popular (nothing like Misha is).

Otherwise I can't see why they'd add Pellegrino to the credits this late in the season unless it was simply a billing demand.

I think it's probably this. Mark gets a lot of TV work - I suspect he's been enticed back with a higher billing. But...I'm only speculating. I honestly have no idea where they're heading. More and more characters I suspect as the Js have bigger family commitments now (but probably still get paid the same as they used to...)