After 10 hours it's incredibly hard to unlock any more cards and then you got to start spending a bunch of money. It's incredibly hard to also find the right card pack that you want with the certain cards that you wish to play with. And by the time you unlock that pack you'll most likely be out of gyms and there's a 23 hour time limit on every pack you unlock so if you don't buy it within 23 hours it's gone! So basically spend an eternity trying to unlock gyms or whip out that wallet.