It's good for surprise factor. I run Specs Unnerve T-Tar sometimes, and it works well to fake out the opponent. If you have a CS team or a Sun, Rain, or Hail team, you can use this Pokemon to make your opponent expect Sand(for CS teams that have the outside appearance of Sand Teams), or keep your opponent guessing which of your possible Weather Setters is actually going to set it up.
But it should be used with caution, because after the surprise factor is over, there isn't much use for it.