Allow me to first admit that I'm not a fan of "Swain is a puppet" theories. I find that it takes away so much depth of the singular individual. Besides, Swain can still summon Beatrice while in Raven form, displaying that she still exists outside of Swain's "True Form".
I like to imagine he's in a situation similar to Elise. He made a far more long term deal than Elise, and made a much larger PERSONAL sacrifice. As such, he can't survive, long, without the Raven Magic to sustain him. The bird is his "raven goddess" if you will.
I do love the idea of Swain and the Raven having a symbolic, albeit Love/hate, relationship.