Instead of the prophet being given another object lesson to show to the people, Ezekiel is asked a question by God in regards to the usefulness of a vine branch. What purpose does a vine branch serve? The only purpose is when it is connected to the primary vine and able to produce fruit. If you take a vine branch off it is good for nothing. You can shape it or use it to hang things on, it can't really be used efficaciously as fire wood. A vine branch by itself serves no purpose. Only when it is attached to the vine does it serve a purpose and that purpose comes from it being attached.
Since Jerusalem has chosen to separate itself from God by worshiping idols, then they are useless. They serve no purpose. The people of Israel without their vine, God, serve no purpose. As a result we see in vs.8 that God "will make the land desolate, because they have acted faithlessly." It is a reminder that every time that we turn our back on God we have become a useless branch that serves no purpose. Stay connected to the vine.