How do you not "leave a backup option exposed"? By recruiting them until they consider you?
If that's the case, then if you don't end up needing them, you have to effectively "waste" the money you need to spend in order to do get them considering you, which could potentially cost you any battles you're fighting (which you hope to win so you don't need the backup player).
Plus, if you get your backups to consider you, then you end up in a scenario where human coaches can see that you have more players considering you than you have scholarships to fill, which some coaches use as a signal to try to fight you in battles.
So now you have your backups considering you to not "leave them exposed", but you end up in more battles for your primary players and end up losing more of them, so you end up with backups rather than primary players in several spots. That's a possibility with that strategy.