Heaven (or at least Zachariah's guys) and hell seem to have some kind of deal. Or at least they're cooperating enough to let the other side get their hands on their own Vessel so they can get this Apocalypse on the road.
Logically, you'd think that Heaven would do any effort they can to make sure Michael gets his Vessel before Lucifer gets his. Lucifer's less powerful while in Nick, than he'd be in Sam after all... Or at the very least you'd expect them to try and destroy Sam before Lucifer gets his hands on him.
Yet instead, they've revived Sam at least twice now.
On the other side, you'd think that hell would be happy that Dean is saying 'no'. As long as he does, and Lucifer already has a Vessel, temporary one or not, they have the upper hand. Yet standing orders from hell are to kill Dean Winchester.
This might seem logical until you realize that as shown in 516, once Dean dies, he goes to Heaven...and right into Heaven's hands. In fact, at this point, it would be better for Hell to keep Dean alive, because it keeps him out of Micheal's hands.
Strategically, both make no sense, unless you accept that both sides seem to have a deal to assure as much as they can that both sides are wearing a Winchester for the Apocalypse. And that to me is rather scary.