Sam and Dean became redundant with the introduction of Castiel/Crowley

Does anyone else feel that Sam and Dean are totally pointless in regards to defeating the big bads of the show? Now, lets say post season 5, I mean Castiel is an angel who can travel back in time in the blink of an eye, why would a being so powerful rely on the strength of two humans to defeat a God-like adversary? Same with Crowley, he often teams up with the brothers to defeat a common more powerful enemy, but why rely on two humans when he has a legion of demons at his disposal? I get it, the show is about the brothers so obviously you need them for the plot, but with the introduction of Cas and Crowley who are more than capable to take on these larger threats the Winchesters themselves are quite redundant.