Cas's powers work/don't work for the same reason Deanna Troi was ineffectual most of the time on ST:TNG, and why female characters on TV and in comics (pre-Buffy) had powers that worked unpredictably: he isn't the lead. If his or Deanna's powers worked the way they're supposed to, most of the shows would have been over in the first ten minutes, and then what would the heroes do? It's like having Superman in the Justice League. If you've got Superman, you really don't need the rest of the Justice League. (Except for Batman. We'll always need Batman.) If Castiel's at full power, you really don't need Sam and Dean.
Which is why I would totally love Cas to be the one to take on Chuck.
It would play beautifully on the meta themes, because Cas isn't supposed to be here. He's that character all writers both pray for and dread: the one who shows up on the page unexpectedly and takes on a life of their own. Cas was never meant to survive beyond Season 4; he was supposed to be gone after maybe four episodes. But he and Misha proved so popular they kept him around. Now here we are eleven years later and he's still on the show, still part of the storyline.
Which makes him Chuck's worst enemy. He's the spanner in the works, the fly in the ointment, the monkey in the wrench. No matter what the writers do to him, Cas survives. He's too popular to kill. His own creator can't control him. He has never, ever done what he was supposed to.
Let Chuck win. Let him have the boys kill each other. Doesn't matter. Castiel will figure out a way to screw it up. Be great if he tricked Chuck into restoring him to his full angel glory. Then we could get the ending I postulated last season: Cas hops back in time and pulls off the retcon of the century by smiting Yellow Eyes before he can contaminate Sam or kill Mary. Time travel and creating alternate timelines worked for the Avengers, it'll work here. If Chuck doesn't like it, too bad. He lost control of his story years ago. It belongs to the fans now. If they don't like it, they're create their own versions and bring the boys back. (Sam and Dean as tulpas. There's a thought.)
Imagine if Cas and Chuck actually had this conversation on the show. Meta enough for you?
Or the show's writers could follow standard procedure and have the boys pull something out of their butts at the last minute (i.e., the final three episodes) and kill God. Then ride off down the road, I guess, until they make a theatrical movie a la X-Files or reboot in five years. Hopefully with a better crop of writers at the helm.