It was not used lightly. Its purpose was not to label a person, thus turning other believers against them. In fact, after all Israel and Judah had done, time and again, God asked them to come back to him and return. He promised to accept and heal them. It's sad to see that God accepts and wants people back while some Christians and churches do not.
Several things can occur.
There have been ministers with pompous attitudes who have declared from the pulpit that they are glad someone no longer attends their church. They would not want to have them back. I am afraid that these people have forgotten some very basic concepts of Christianity. God is in the process every day of restoring people to himself. Unfortunately, some Christians do not see this, nor do they help with the process.