it's almost like I looked at my characters and went "Cas would be useful for the defenders to have, and it would be really horrible if El were a Defender at great risk of death" and then cackled like an evil person.

Nah, he's not that silent. If he flies, his wings (and clothes) rustle. His shoes make sound. BUUUUUUUUUUUUUUUUUUUUUUT he can also cloak himself and go invisible and then not make any sound. That'd burn through his lantern real quick tho.