I agree that the introduction of XP rewards would somehow compromise the approach that the Steemonsters card should be scarce and valueable in the long run. The combining mechanism should be the only method to level up cards. That said doesn't mean that we shouldn't consider a second income stream besides Card Packs rewards. I could see a system based on SMT that allow players to get "Game Credits" which are worth something in the open market. These Game Credits could be used to purchase more cards on the open market. This would nether inflate the BCX of Cards nor would it inflate the amount of packs given out. It something like peakmonsters did with introducing Credits for purchases its just another little incentive to purchase more cards.
I'm looking forward to this discussion and its crucial to the further development of Steemmonsters.