I was not a believer in the Dean as Cas's vessel theory, but... while in purgatory Benny told Dean he knew how to open a door to let a human out, but the door was only for humans. Dean carried Benny's soul within himself and returned it to Benny's bones.
What if that was the only way Cas could leave? Because both Cas and Dean had been sent there intact -- souls and bodies, Dean had to 'kill' Cas to free his soul. Then, he carried Castiel's soul out with him, too. However, since there were no bones, he either hasn't released it (which can explain his eating habits) or he did free the soul but it had nothing to return to so it 'disappeared' making Dean think Castiel was truly dead this time.
Either way, I don't think Dean would leave purgatory without Castiel. Dean's a no man left behind kind of soldier.
So what do you think?