In the best of all possible worlds, icons don't need labeling. In the real world, a large portion do. Which brings-up the question of "why do we need icons at all?". And one could argue that often we don't.

For all its flaws, Microsoft Metro has the merit of realizing that icons often don't really have the communicativeness that many designers imagine. Sometimes a word is worth a dozen very sketchy pictures too, or at least is more specific than any given picture.