The DH questline is unique in the sense that the final reward is irrelevant (you don't want to spend 5-mana to play a 7/7 that does nothing so that the last 3 cards left in your deck by that point cost 2 less), but the intermediate rewards are much more powerful than those of any other questline. Il'gynoth OTK DH runs the questline just for the first two rewards - the mana reductions. It also allows a formerly horribly bad card like Glide to become playable again and not having the possibility to outcast a Skull is no longer a game-deciding factor.
The DH questline is unique in the sense that the final reward is irrelevant (you don't want to spend 5-mana to play a 7/7 that does nothing so that the last 3 cards left in your deck by that point cost 2 less), but the intermediate rewards are much more powerful than those of any other questline. Il'gynoth OTK DH runs the questline just for the first two rewards - the mana reductions. It also allows a formerly horribly bad card like Glide to become playable again and not having the possibility to outcast a Skull is no longer a game-deciding factor.
Only 5? WoW that's an improvement