This irritation others have over no rewards in Dungeon Run makes me laugh. Do you get rewards from most games that you beat? Usually if anything maybe an achievement unlocked. The card back serves that purpose. Further, Dungeon Run costs $0.00 I just don’t get the frustration.
The other thing is, when I log into HS, the first thing I see in quests which are irrelevant to me. A few dungeon quests would add a fun challenge and make the entry into the game feel less irrelevant.