I understand that he came back. But what if he had not? He made the choice to take his inheritance and leave. Of course his father wouldn't disown him and would still consider him his son. But he did give his son the choice to leave ... even risking the fact that he may never return. Yes, we are God's children and always will be, but we are free to leave Him and choose the wrong path.
Perhaps the parable also tells us that, ... though they may stray for a season, ... in the fullness of time, God's children will always come back home.
SO... IF you stray... have fun! Live it up, because you ain't gonna die till after you come home?