Although it may be the case that unclean spirits (i.e., demons) of the first century operated in the manner in which Jesus described here, that was not His purpose in speaking. Our Lord's emphasis was not upon teaching a lesson concerning a possessed individual but rather about "this wicked generation."
The demon who had been displaced cannot seem to find any "rest" once outside his former residence. Interestingly, the unclean spirit still claimed the man as his property (i.e., "my house"). What does Jesus mean by saying that the demon would find his former abode "empty, swept, and put in order" when he returned to it? The idea being communicated here is that although the man who was freed from the demon looked good on the outside, his inside was empty and had not been filled up with anything of real value or substance. Although there had been a formal reformation, no lasting change had taken place in the man's life.
Once the unclean spirit observes this, "he goes and takes with him seven other spirits more wicked than himself". The plan of this demon was to seek help in order to regain entry and be able to maintain it. No one would question that "the last state of that man is worse than the first." Although no one would desire to be possessed at all, certainly it is common sense to understand that being possessed by one demon is better than being possessed by eight of them!
Jesus made His application at the end of Matthew 12:45 - "So shall it also be with this wicked generation." Let it be understood that Jesus was discussing two conditions experienced by one "man", or more correctly, one "generation." The last state of that generation would be worse than the first. The reference appears to be to the continually increasing wickedness of the Jews, which ultimately resulted in the dreadful scenes related to the destruction of Jerusalem in 70 A.D. Yes, they may have done (or perhaps were doing) some external, temporary acts of reformation in Jesus' day, but they, as a nation, lacked true repentance and righteousness. Although Jesus could free them from their demons (or sins), the nation, as a whole, rejected Him and would soon allow even more sins to control them. Thus, they tragically ended up worse off than before!
Although it doesn't appear to be Jesus' primary application, this principle is obviously useful on the individual level. In order to maintain a life free from sin, one must not merely expel evil by genuine repentance. He must also fill his heart with good thoughts (Phil. 4:8) which come from a steady, spiritual feasting upon God's word (cf. II Tim. 2:15; 3:16,17; I Pet. 2:1,2). Friends, if you have been freed from the shackles of sin, labor to continually fill your life with good thoughts and righteous deeds. Otherwise, your former master will be successful in his constant efforts at regaining a foothold in your life!