OOC!
He really would be! With a little Winchester training and some people skills, Cas could be a pretty awesome hunter. Sam just doesn't want Castiel to get mixed up in being a cop because there are so many different things that could go wrong, lots of which would probably end up hurting Castiel in the end. Hurt Cas is bad. Sam is not a fan of that, especially since it's become pretty clear to him that he's already struggling enough on his own as it is. The last thing he needs is more badness on his plate!
Juliet is being awesome right now. She is definitely on point RE: Castiel and Sam needs to listen to her!
Sam is just...in a very strange place. He's holding it together for now, but he's pretty resigned to the idea that this apocalypse will end badly for him. Because of that and the idea that he's got a lot of people he needs to be looking out for, Sam is pretty fixated on GETTING THINGS DONE at the moment. Which means throwing away Fun Sam and trading him in for Serious Let's Research 24/7 and Hope Lucifer Doesn't Kill Us Today Sam. Combine that with him being convinced that this whole mess is his fault to begin with and...um. Strange place!