Holding justification and adoption together is supremely important. What happens if we fail to hold both doctrines together?
Justification without adoption can lead to a static and impersonal understanding of God. Yes, you will be firm in your understanding of forgiveness, but you will know nothing of sonship. When other people talk about a relationship with God, it sounds weird to you.
Adoption without justification is both impossible and frightening. I have met many Christians who understand that they are a son or daughter of God, yet they know nothing of justification. The danger with this mistake is that it leads to being a person who is always afraid of their heavenly Father. Security is only a solid as today's performance or failure.
Fortunately for us, in Christ we have both!