In The Conjuring we only get to see what happens to Bathsheba, but what happened to the other demons in the film? Are they harmless and still roaming in the house?
In the film, Bathsheba was a wicked woman, who killed off her child to please the devil. Other ghosts or apparitions as seen in the film are actually direct or indirect victims of Bathsheba.
Bathsheba had cursed people who took or lived on her land. So even after she was killed, she continued to possess mothers and made them kill their children. So the other demons which you refer to are actually NOT demons, but the ghosts of the mothers who have committed suicides after Bathsheba made them kill their children. Also, Rory is one such murdered child killed by a possessed mother.
Bathsheba is the only one who wants to hurt the family, and possess Carolyn to make her kill her daughter. All the other ghost spirits are not actively tormenting, merely living their horrors in the afterlife. It was only Bathsheba who needed to be exorcised, while the other ghost spirits could have been ridden off with other means. Bathsheba was the danger to the family.
Links: The Conjuring - Wikipedia