I don't get why the commentators I've consulted think this passage on the unclean spirit is a parable. It strikes me more as a truism. A person who has had a demon cast out of him isn't safe from repossession unless he receives the Holy Spirit in place of the unclean spirit. Otherwise he is just a sitting duck, still on that demon's radar screen. And if the cast-out demon wanders around and finds no other place to go, he will surely check back to see about the old place; and if the coast is clear he will bring heavy reinforcements (seven other more wicked demons) to make it harder to exorcise him next time. This is just a demon being smart, self-serving and practical. We have a lot to learn from this.
It's not enough to enjoy the benefits of Jesus' ministry, you have to embrace him wholly; otherwise you'll find yourself in a worse state than if you had never been exposed to the truth in the first place. A few verses back Jesus had said, "He who is not with me is against me, and he who does not gather with me scatters" (v. 30). You belong to either God or Satan. A person who has had a demon cast out of him can't remain empty like an unoccupied, swept house. An unoccupied house is waiting for someone to occupy it, the only question is who. If, after being graced by Jesus' presence, you don't receive him by faith, you will be overcome by a greater, more damning unbelief than you had before. A demon who loses his grip on a soul temporarily will only redouble his efforts to get it back. Unless you have the Spirit of God himself dwelling inside you ready to take him on, you will be defenseless.
Obviously not all unbelievers are demon-possessed in the way we think of demon possession, but they are under Satan's domain. Jesus' teaching shows that there really is no such thing as an empty house, a place of spiritual neutrality. Those who aren't controlled by the Holy Spirit have another spirit at work in them, breeding unbelief, pride and fleshly desire. It is a warning to those who think they can sit on the fence when it comes to following Jesus. That edge of the fence that you think you are perched upon doesn't really exist. And if you fail to embrace Christ, in spite of all the ways he has graced your life, you will fall so far down the side of unbelief, you will never find your way back the opposite way.