No, I don't think they should add Benediction/Anathema or Lok'Delar/Rhok'Delar back into the game. I don't know about Benediction (made my Priest in BC), but for Lok'Delar it was incredibly difficult to get. Probably the hardest quest I've ever done in WoW, in fact I'm sure it was. I'm assuming the Priest quest was relatively difficult as well. And if they add the items back in I am 100% sure the quests will be far easier than they used to be, and it will make the item no longer special when everyone runs around with it.
Imagine the green fire quest in the next expansion if nothing changes, when everyone has it, it won't be cool or interesting anymore like it is now. If they remove the green fire when the next raid tier hits or when the next expansion hits then it'll still have meaning and be cool and interesting when you see people with it like it is now.