This is how almost every collectable card game works — some cards are more rare than others. At some point, players start getting too many duplicates of some of the more common ones — more copies of one than will ever be useful. The common game design pattern to solve this problem of generating “useless cards” is using the extra duplicate common cards to build up other cards which are more rare.
Example: Hearthstone has you open card packs for coins. At some point you start getting too many duplicates of the same common card — more than you can use. Hearthstone’s solution is to have you “dust” the card so you can build towards some rarer card you don’t yet have.
If you don’t have a system like this to solve the problem, when you open your Recruitment chest, sometimes you’ll be getting a single card out of it which will be completely useless.
Yes, you could make a game design where you turn these extra cards turn into something like a totally different currency from generating other cards you don’t yet have (eg coins.) We didn’t take this path for the same reason Hearthstone and every single other game I know of didn’t do it: It would probably wreck the game economy. I wouldn’t even know how to model it — I don’t think we want to be the first game out there to try this when there is a standard game design pattern.