Virtue is its own reward
I recently found myself killing these guys in WoW. They’re basically not itemized at all, and while they certainly look like ugly doggies, they can’t be skinned either. (Read the comment thread at that link to see more people bitching about their itemization.)
This made me realize — by way of contrast — just how good the reward feedback is in WoW, and in the expansion specifically. I was killing the mobs, getting reasonable xp, had very little risk, and they were in plentiful supply. And I was completely unsatisfied.
Just down the road from them are these guys, and if you look at their drops, you’ll see ‘Mark of Sargeras’ and ‘Fel Armament’. Both of those are faction reputation turn-in items; the latter is converted into a kind of currency you can use to buy enchantments. Other than the faction items, they generally drop more junk loot per kill than the Hounds. They’re similar in risk, xp, and supply. And I could kill those guys all day long.
When you add a quest that asks me to kill them, I feel like I’m engaging in a fundamentally different activity than ‘mob grinding’. And in those rare instances when I have two or more quests asking me to kill the same mobs, I barely even notice the passage of time; the same basic activity becomes totally engrossing.
This suggests that all the time spent writing content in the world isn’t as compelling as adding another reward track. In an imaginary world where all resources for development are fluid and mutable, it’s better to spend a man-hour adding an additional way to offer rewards for killing 10 mobs than to spend that same hour creating a quest that breaks the mold of ‘kill 10 mobs’.