Understand it uses top 30 cards to determine a good match value -
Like many in growing alliances - I don’t have hundreds of cards - and often keep duplicates I’m lucky to pull - like HotM, 5* that might be buffed later, new features down the line may give them value. etc.
If there was a small threshold I had to reach - like say 1.20 or even 1.02) for a card to be useable in war - these spare cards wouldn’t count and wouldn’t hurt me … and I could easily choose to push them up slightly to opt in
The opt in is at an account level - and assume opting in individual cards would be a nightmare … this seemed easiest.