Well. technically, angels have to have a vessel say "yes" and can't inhabit a corpse, so I can see Crowley bringing Nick's soul back just so he'd say yes. I mean he did knowingly say yes to Lucifer the first time, so his soul probably didn't go to heaven when he died and Crowley would have had access. All of that would mean that the writers actually remember canon from season five, and there is no indication that they remember canon they actually wrote a couple of seasons ago, I'm looking at you "cosmic consequences".

I am guessing they are providing an alternate vessel for Michael and maybe someone to bond with Sam for a short while. It looks like they decided that Cas doesn't have much to do, so Sam will lose the bond with Jack, and Cas will take up the "real father" role with Jack. At least that's where I think the Cas/Jack interaction was heading.