I believe the point is that NPCs can effect the gameplay mechanics of classes.
I think the more important thing about Netherkurse is that his existence proves that there are melee Warlocks in WoW.Talen's argument relies on the premise that Nethekurse can do everything a Demon Hunter could. There's no premise for that within his design whatsoever that tells us this.
If a Warlock is capable of melee, then we have our Demon Hunter. A DH is nothing more than a melee Warlock.