Excuse me? I'm sorry, but I have to say that Xerath's passive is quite beneficial. I can't count the number of times a Garen, or Talon, or Pantheon, or some other AD champ thought I'd be an easy kill and I turn around and kill them with just a few hp to spare.
I'm quite defensive against calling xerath's passive worthless. Because if your opponent manages to close on you, that armor still helps a ton on not being worthless.