I personally don't see the problem, what you trybto get from campaign and dungeons are two different thing.
> Campaign you generally try to farm XP and silver
> Dungeons you generally try to farm gears.
You could try to farm gear in campaign, but the gears there are generally of low rarity, hence not really desireable, plus the gear set available in campaign are severely limited
Vice versa you can farm XP in the dungeons, but they're definitely of significantly lower amount.
So you're comparing apples and oranges here if you compare Nightmare campaign stages vs Dragon Den Dungeon stages..