Jimmy's body is a voice. He is a pair of wings. Castiel finds freedom on earth through Jimmy's sacrifice. He can stretch, can roam as far as he can see. He's not a wavelength anymore. He's been made real.
Castiel can interact with humanity. He can look after Dean, speak to him - touch him.
But the world has other plans.
Castiel's brothers are closing in on them, and no matter how much he tries, Jimmy cannot be a shield. "I'm sorry," He whispers, when his body is not strong enough to stand up to Raphael.
"There's nothing to forgive." Castiel assures him, because Jimmy's body is perfect as god designed it. He loves this body - his freedom, his free will manifest.
Perhaps that is why God brings him back: a reward for finding the beauty in his creation that the other angels seem to have forgotten. Jimmy is his miracle, and they both rejoice in it.
For the second time, they are destroyed. For the second time, they are restored. Castiel runs back into Jimmy's embrace, and it's like coming home.
Only this time, the world is not the same. Dean lies on the ground, broken and bloody, staring at the place where his brother will never return. Castiel can heal him, he can even bring Bobby back to life, but he doesn't know what to say.
He asks Jimmy what words there are to ease this pain, because he feels for Dean like his heart has been ripped from his chest. If he was formed solely from celestial intent, he would wrap Dean up in his light and soothe this away, but he can't. Not anymore.
"I'm sorry," Jimmy whispers again as Castiel watches Dean leave. He'll go to Lisa, who's body is hers to freely give. She'll wrap Dean up in her arms the way Castiel can't, and tell him the things that Castiel wishes he had the words to say.
For the first time, Jimmy is a cage.