By unlocking the SB cards, does it mean you can now buy/sell it? I think the answer to this is a yes but I just want to be sure.
If yes, I think it's also good to have a card that is permanently soulbound and will not be unlocked/sellable ever until the end of time. Why? Think of this as an achievement that you can use in game. Instead of having the titles that don't really impact the game, a card that you can ONLY get by grinding the game makes grinding worthwhile and an achievement.