The issue is one of literal spirit possession, which in the context of the words of Jesus as exorcist make complete sense. In the context of Jesus as moral guide they seem less clear, or rather the analogy becomes stretched, when you become aware that JWs interpret it to mean that a person is in a vacuum-like state if they have not filled their bodies with the "soul" of pure worship, i.e. the body is a temple. The reason this does not make sense from a JW perspective is that it seems to acknowledge possession while rejecting (in other dogma) the notion of soul/body dichotomy. Yet, the problem of God's breath (ru'ach) persists, and this apparently goes into and out of people freely when they're alternately alive or dead, while yet an individualized soul does not exist. But think about this...if everyone is endowed with God's ru'ach while living, it isn't God's ru'ach which is being corrupted/overtaken by the unclean spirit, is it? If not that, then what?
In Jewish thinking of the first century, unclean spirits are apparently vying with their clean counterparts like battling swarms of locusts to inhabit people's bodies, and if you're corrupt on the inside (cf. Jesus' analogy of the Pharisees to white-washed graves) then it's as if you have a "For Rent" sign on your body for the demons! Unclean spirits were likely used to explain common tragic human woes, like mental illness, etc. Clearly, the depressed were not memorizing enough Scriptures!