Crowley isn't stupid and he knows that if the Leviathans win that the soul smorgasbord that he enjoys is going to go away. The season is drawing to a close and the Winchesters are no closer to figuring out the Leviathans than when the season started. They can hurt them, but there is no real way to kill or detect them.
If they want to resolve this by the end of the season, then they are going to need one heck of a Deus Ex Maquina. My guess that's going to come either in the form of Castiel, who is set to appear in the next new episode, or Crowley. It's probably going to be a mix of both. If the Winchesters are going to defeat the big bad, they are going to need some serious help.
That means either finding a way to kill them or sticking them back in purgatory. Crowley has been way to quiet lately, so when he does come around, which will be soon, he's going to be carrying some hefty firepower. The boys have a personal reason for wanting the Leviathans dead, but for Crowley it's all business.
I haven't been to thrilled with the season so far, but something tells me the final battle is going to be a doozy.